BANGKOK - Tanks and armored vehicles rumbled across Bangkok on Sunday as Thai premier Abhisit Vejjajiva cracked down on protesters leading the most serious challenge yet to his four-month rule.

One day after wrecking a summit of Asian leaders, demonstrators fired into the air and attacked his convoy as he was being driven away from the interior ministry where he had minutes earlier imposed a state of emergency in Bangkok.

Abhisit said he was safe and unhurt after the incident, and called for calm while threatening the use of force to restore order.

The latest escalation in the country's chronic political turmoil came after police arrested the leader of activists loyal to ousted former prime minister Thaksin Shinawatra who had targeted Saturday's summit.

Colonel Sunsern Kaewkumnerd told AFP the army, navy and air force had been deployed to ensure the security of public buildings, junctions and transport hubs at 50 spots in Bangkok.

Military and police reinforcements were also seen arriving to protect the Royal Palace compound.

Earlier, Abhisit's deputy Suthep Thaugsuban appeared on national television to appeal to soldiers and police to enforce the emergency measures, amid signs they were reluctant to intervene.

"Police and military -- you must carry out your duty to your best ability and restore normalcy as soon as possible," said Suthep, who is responsible for implementing the measures.

"These are not constitutional demonstrators. They have injured senior officials."

The army has generally shied away from confronting protesters since action against riots in 1992 left dozens dead, and police moves against anti-Thaksin demonstrators last October left two dead.

As tanks and soldiers fanned out, television showed red-shirted protesters armed with sticks and paving slabs smashing a car they mistakenly believed was carrying the prime minister and a separate vehicle carrying Suthep.

It is the third time in eight months that authorities have imposed a state of emergency in Bangkok.

The move bans public gatherings of more than five people and empowers the police and military to detain suspects for up to 30 days without charge.

However, police said the rally outside Abhisit's offices swelled to 15,000 after the declaration, with its leaders sounding a defiant tone.

"I don't care for this state of emergency, it's just paper dirtied with ink," protest organizer Nattawut Saikuar told the crowd.

The demonstrators' political hero Thaksin later addressed them by telephone from exile.

"I will closely monitor the situation and if there is any violence I will return to Thailand immediately. I will not allow them (authorities) to use force," Thaksin said, urging army troops to join his movement.

Pro-Thaksin protesters took to the streets two weeks ago demanding Abhisit quit. They say he came to power illegitimately through a parliamentary vote in December after a court forced Thaksin's allies from government.

The chaos in Bangkok is a virtual replay of crises last year that ended up forcing out two premiers loyal to Thaksin, who was toppled in a military coup in 2006 and remains in exile.

Abhisit is under intense pressure to curb the unrest after the humiliating cancellation of Saturday's regional Asian summit, after which authorities were forced to evacuate foreign leaders, some of them by helicopter from the hotel roof.

The weekend summit was supposed to discuss the global financial crisis and North Korea's rocket launch and included the leaders of China, Japan and South Korea plus 10 Southeast Asian nations.

Police said they earlier arrested the former pop singer turned protest leader Arisman Pongreungrong at his Bangkok home "on the charge of inciting protesters to kidnap the prime minister and cause unrest in the country."

Thaksin remains in exile to avoid a two-year jail term for corruption, but has rallied supporters -- and incensed the government -- with almost nightly video and telephone speeches from an unidentified foreign hideout.