ISLAMABAD, Pakistan — Police manned roadblocks and rounded up supporters of a former prime minister, Nawaz Sharif, who left London yesterday after seven years in exile to return to Pakistan and lead a campaign to topple the country’s American-allied military ruler.

Before leaving London, Mr. Sharif warned that President Musharraf’s government might try to arrest or deport him. At the last minute, he asked his brother Shahbaz Sharif to remain in Britain so that he can lead the party in case anything happens to the former prime minister.

The Pakistani government has hinted it may arrest or deport Nawaz Sharif when he arrives, and an arrest warrant was issued against his brother last week in connection with a murder case.

Any attempt to arrest Nawaz Sharif, whose elected government was ousted by General Musharraf in a 1999 coup, is likely to sharpen political tensions ahead of presidential and legislative elections and could trigger street violence. The looming showdown could further weaken General Musharraf’s faltering grip on power and comes as the country battles surging Islamic extremism that has spread from the Afghan border, where Osama bin Laden and other Al Qaeda leaders are believed to be hiding.

“We are going, nothing can stop us,” Mr. Sharif told reporters at London’s Heathrow airport before boarding the Pakistan International Airways flight. “We are happy we are going go back to our country after seven long years in exile.”

He said he expected General Musharraf to invoke emergency powers to stop his supporters in Pakistan, but he said that only underscored the need for him to return and fight to restore full democracy and the power of the judiciary.

“More than 2,000 Sharif supporters in Punjab have been detained in a crackdown over the past four days, and others have gone into hiding, said Ahsan Iqbal, a spokesman for Mr. Sharif’s party. Police and security officials, speaking on condition of anonymity because of the sensitivity of the issue, confirmed about 700 arrests. Yesterday evening, police patrolled and manned roadblocks on the main roads leading from Lahore to Islamabad. One mobile squad swooped down on three buses carrying suspected Mr. Sharif supporters. Armed officers mounted each bus and ordered the drivers to take the passengers away — apparently into custody. Police also set up checkpoints in and around the capital. News reports said authorities planned to prevent anyone traveling to the city’s international airport this morning unless they were booked on an outward flight.

“It’s extremely important to show that people are sick and tired of this dictatorial regime,” said Zulfikar Ali Khan Khosa, president of the Punjab branch of Mr. Sharif’s party, predicting huge crowds would travel to the airport despite the crackdown.

Analysts say Mr. Sharif’s return could crank up the pressure on General Musharraf and upset talks on a power-sharing pact with his longtime rival Benazir Bhutto, another exiled former premier plotting a political comeback.