BANGKOK, Thailand — Troops and tanks guarded the streets of Bangkok last night after the army chief launched a coup while Prime Minister Thaksin was out of the country.

Lieutenant General Sondhi Boonyaratkalin said he was acting on behalf of the nation’s revered King Bhumibol Adulyadej.

Some feared that clashes could break out between army factions, some loyal to Mr. Thaksin and others to the king. People on the outskirts of Bangkok said they saw 35 trucks each with 10 armed members of the Queen’s Regiment, the equivalent of the Brigade of Guards, traveling toward the city center.

Tanks guarded the entrance to Government House, a sprawling Italianate building, and two others were stationed nearby as soldiers lined up along the walls of the compound.

A general said the deputy prime minister and the defense minister, two of Mr. Thaksin’s closest allies, had been arrested.The Associated Press reported that it was the first coup in Thailand since 1991, when an attempt by Prime Minister Kraprayoon, a military general, to retain power was later countered by mass street demonstrations and Mr. Suchinda’s ouster. After that, the military vowed to remain in its barracks, in contrast to earlier decades when military coups were a staple of Thai politics.

A witness to what seemed like a classic coup described the atmosphere as “very calm,”as about 100 civilians milled around “smiling and taking photographs.” Mr. Thaksin, 57, a telecoms tycoon who came close to acquiring Liverpool Football Club two years ago, has caused resentment in the army by making hand-picked appointments and is accused of policy failures that have ignited a separatist insurgency in the predominantly Islamic south.

A senior military officer said on television and radio that the constitution, Cabinet, and Parliament had all been suspended and that martial law was in force in Bankgok.

He said a council for political reform, with the king as head of state, had seized power in the capital and neighboring provinces.

“There has been no struggle,” he said. “We ask for the cooperation of the public and ask your pardon for the inconvenience.” The statement emphasized that the coup was temporary and that a commission would be set up to decide on political reforms and oversee an election. Officials said General Sondhi and other military leaders had met the king at the royal palace, apparently to work out an interim government. General Sondhi said: “The council found it necessary to seize power as of now.”

Mr. Thaksin, whose opponents accuse him of corruption and abuse of power, was in New York, where he was due to address the United Nations General Assembly. Television news showed him saying that he was sacking General Sondhi and declaring a state of emergency in Bangkok. He ordered troops to follow only “legal orders,” but the screens went blank as he was speaking. Most international television stations, including the BBC and CNN, went off the air, and the country’s six public broadcasters flashed a continual message that forces loyal to the king had taken control “to maintain law and order.” Images of the king were shown repeatedly.

Thailand has been in crisis for months. At times, as many as 100,000 people have demonstrated to demand the removal of Mr. Thaksin. In April, he called and won a snap election, but the result was annulled after allegations of cheating by his Thai Rak Thai Party. He has been serving as a caretaker prime minister since then.

Thailand has a long history of military coups since World War II, but the last was 14 years ago, when dozens died as security forces opened fire on protestors.

Although last night’s events appeared bloodless, and the capital remained peaceful, the situation represents a climax to a power struggle that has been building up for years between Mr. Thaksin and Thailand’s elite.