Along the dusty roads, and through the rural towns and villages of the poorer parts of the Islamic world, travel small groups of men from a secretive and little-understood movement called Tablighi Jamaat, carrying what its followers proclaim to be the true word of God.

In so far as it speaks at all to outsiders, the organization, founded almost 80 years ago, declares itself to be nonpolitical and nonviolent. Yet, with increasing and alarming frequency, the name of Tablighi Jamaat is cropping up in the worldwide fight against terrorism.

Several of those arrested on August 9 in connection with the alleged plot to blow up airliners en route to America from Britain, had attended Tablighi study sessions in Britain. At least two of the British suicide bombers of July 7, 2005 — Mohammed Siddique Khan and Shehzad Tanweer — had worshipped at a Tablighi-run mosque in Dewsbury, West Yorkshire. The failed British shoebomber, Richard Reid, is known to have Tablighi associations, while the path to violent radicalism of John Walker Lindh, now serving 20 years for treason, appears to have begun with his contact with Tablighi missionaries. In America, the activities of the Tablighi have been under close scrutiny for some time. A confidential FBI memo, leaked to a television news network last year, portrayed the group’s followers as likely to be particularly susceptible to the terrorist cause.

“We have a significant presence of Tablighi Jamaat in the U.S., and we have found that Al Qaeda used them for recruiting, now and in the past,” the deputy chief of the FBI’s international terrorism section, Michael Heimbach, said.

While rival Islamic groups flaunt their reach and power, Tablighi Jamaat — loosely translated as “propagators of the faith” — exercises influence in a more discreet, yet more worrying, fashion. The organization was founded in British-ruled India in 1927, at a time of growing political friction between Hindus and Muslims that led to the partition of the sub-continent in 1948.

Both communities rallied their faithful loudly, and one of the voices that rose above the clamor was that of a scholar and cleric who prescribed a strict code of religious observance, Muhammed Ilyas Kandhalawi. Tablighis, however, were taught that their true security — indeed, their religious duty — lay in recruiting as many followers as possible.

No limit was placed on the potential pool of converts, and, implicitly, the ultimate objective was the Tablighization of the world. The group, for all the mystique that surrounds it, has been diligent, and, today, with a growing presence in the West, it is viewed by anxious critics as a Trojan horse of Islamic fundamentalism.

It operates legally in both America and Britain, and none of its leading figures is known to have said anything that suggests support for terrorism.

Indeed, the Tablighis reject any form of political alignment, restricting their activities, according to the group’s founding creed, to prayer and self-improvement through intense study of the Koran. So much so, that some hard-line Islamic groups have, in the past, attacked Tablighi Jamaat for its conspicuous failure to take a political stance on issues such as Israel and the Iraq war.

Yet, Western critics say, this passivity is not all that it seems. The group’s ideal of a world governed by an ultra-conservative, neo-medievalist form of Islam, in which women are subservient and all laws are based on religious dictates, is barely distinguishable from the wish-lists of Al Qaeda and the Taliban.

The head of the School of Indian and South Asian Studies in Paris and a Western authority on Tablighi Jamaat, Marc Gaborieau, says the group’s objective is “the conquest of the world.” Less easy to divine, he confesses, is the strategy.

“It is extremely secretive and suspicious of outsiders, and no one at the center of its activities has been fully identified or has spoken about how it operates. We know that it does not recognize national borders and that, despite its claim to be apolitical, it does have ties with politicians and branches of the military, particularly in Pakistan and Bangladesh,” he said.

The modern leadership of the Tablighi is one of its core mysteries. After Kandhalawi’s death in 1944, control passed to his son, Muhammed Yusuf, who led a dramatic expansion across the sub-continent until his own death in 1965. It is understood that real power is still held by family members, although how it is exercised remains largely unknown.

“It operates in every sense as a secret society in this country, as much as elsewhere,” the director of the Institute for the Study of Islam and Christianity, Patrick Sukhdeo, said. “Its meetings are held behind closed doors. We don’t know who attends them. How much money it has. It publishes no minutes or accounts. It doesn’t talk about itself. It is extremely difficult to penetrate.”

For all its growth, Tablighi Jamaat has, in one sense, changed little. In the early days, it would send its followers out as missionaries. Working in small groups, with few material possessions, they would walk through villages, denouncing modernity as blasphemy and calling for a return to the seventh-century origins of Islam and what Kandhalawi perceived to be the purity of the faith.

Today, the organization runs on a global scale — it has about 50,000 followers in America alone — but its missionaries still operate in the traditional way, visiting Islamic community groups and mosques to call for a re-embracing of the faith.

In Britain, the group is run from the 3,000-capacity Markazi Mosque in Dewsbury — built with funds from Saudi Arabia — which also functions as Tablighi Jamaat’s European headquarters. Signs around it warn: “Photography prohibited. Unauthorized persons not allowed. Trespassers will be prosecuted.” Residential courses for young Muslims are held there, and the group sends its missionaries across the continent.

Last week, a secretary and trustee of the Tablighi Jamaat movement and a spokesman for the mosque, Shabbir Daji, denied the organization had any links with Islamic extremism.”We are a society that offers information to Muslims on how to reform themselves,” he said.

“We are not a political organization, and we do not let any brothers speak about politics within the mosque. We do not create … [terrorists]. We condemn them totally. If we think anyone has an agenda outside of our own, we immediately throw them out of the mosque. We have nothing to hide. We feel very bad and very angry that we are being linked to what is going on. People are putting out information that is untrue. It is not creating a good image in the community, and that is not helpful to our cause.”

Local support came from Hanif Moyett, a councilman for Batley, West Yorkshire, and secretary of another Dewsbury mosque. “Their function is to go out into the community and to invite their brothers who have been led astray to embrace their religion again,” he said. “They encourage those who are not regular visitors to their local mosques to attend more often. They lead them down the path of good.”

The Tablighi appear fully aware of the suspicions that they arouse. An account of a meeting held in Stratford, east London, reported Saturday, told of the group’s belief that it had been infiltrated by informers and that the authorities kept flight records of all members who traveled. One official said: “Tablighi is like Oxford University. We have intelligent people — doctors, solicitors, businessmen — but one or two will become drug dealers, fraudsters. But you won’t blame Oxford University for that. We are not worried. They can close us down, and the effort will continue. We have no fear.”

The group’s ambitions in Britain are, nevertheless, on a grand scale. The Tablighi is in advanced discussions with the London Development Agency for the construction of a giant, 70,000-capacity mosque complex — Europe’s biggest and the centerpiece of an “Islamic Village”— in the east London borough of West Ham.

If it goes ahead — at an estimated cost of between $188 million and $376 million — it will become London’s biggest religious site. The London mayor, Ken Livingstone, is a supporter, as, it appears, is the LDA and, once again, much of the money is likely to come from the Saudis.

Not everyone is overjoyed. “I think, at the very least, we need to know much more about Tablighi Jamaat,” Mr. Sukhdeo said. “Who runs it and what it is up to? And I think we need to ask whether we want an unequivocally ‘Islamic Village’ in London. To me, this is taking us down the path to parallel societies — one mainstream British, the other Islamic — and there ought to be some serious debate about it.”

The concerns go farther. Western intelligence — while exonerating the Tablighi of direct involvement in terrorism — increasingly sees the group as a component of the radicalization of Islamic opinion.

In France, according to an intelligence report cited by Le Monde, up to 80% of known extremists have, at some stage, passed through Tablighi ranks, leading to the group being labeled “the antechamber of terrorism”by intelligence officers.

Certainly, there appears to be a nonetoo-distinct crossover between the Tablighi and recognized terror organizations. Abundant evidence exists that large numbers of Tablighi followers are undergoing training in military-style camps in Pakistan and Afghanistan, while the Moroccan government — in company with others — has published documents suggesting that home-grown terrorists frequently use the Tablighi Jamaat as a cover,”to hide their identity on the one hand and to influence these groups and their policies on the other.”

For its part, the Philippines government has specifically accused the group of funneling Saudi money to Islamic radicals in the south of the country.

When American investigators tackled the case of a Pakistan-born American citizen, Iyman Faris, now serving 20 years in prison for his part in a plot to blow up New York’s Brooklyn Bridge, they quickly discovered that he had posed as a Tablighi preacher in order to have an expired airline ticket re-issued in Pakistan. Such is the group’s reputation in much of the Islamic world that the travel agent readily made the amendments to the ticket.

American counterterror agents depict membership of it as a useful measure of fundamentalist leanings. One official told the New York Times: “It’s a natural entree, a way of gathering people together with a common interest in Islam. Then extremists use that as an assessment tool to evaluate individuals with particular zealousness and interest going beyond what’s offered.”

John Lindh came into contact with the Tablighi a year after his conversion to Islam in 1998. He traveled with a small group of the organization’s preachers and later, when looking for ways to advance his Islamic studies, was guided by a Tablighi contact to a religious school in Pakistan. From there, he moved into the fighting ranks of the Taliban.

American authorities point to numerous similar cases. In few, if any, can the Tablighi be held directly responsible for its followers becoming jihadis, but the group’s pervasive presence and championing of its austere and rigorous form of Islam is widely seen as part of the conditioning process.

“The West’s misreading of Tablighi Jamaat’s actions and motives has serious implications for the war on terrorism,” the vice-president for research at the Institute for Security Policy in Washington, D.C., Alex Alexiev, said.

“Tablighi Jamaat has always adopted an extreme interpretation of Sunni Islam, but, in the past two decades, it has radicalized to the point where it is now a driving force of Islamic extremism.”

The method, Mr. Alexiev said, is simple. After being brought into the organization, promising recruits are typically invited to Pakistan for additional training. There, albeit seemingly without the Tablighi’s knowledge or approval, they are liable to be approached by representatives of terror organizations.