Arisman Pongruangrong's escapade was the latest embarrassment for the government, which less than 30 minutes earlier had announced on national television that security forces were surrounding the hotel to arrest Arisman and other leaders holed up inside.
Authorities have tried without success to end a monthlong sit-in by tens of thousands of "Red Shirt" protesters in some of Bangkok's most popular shopping and tourist districts. At least 24 people were killed last week when troops tried to clear one group of protesters.
Friday's failed crackdown signaled the government was willing to risk another confrontation with the Red Shirts, who are campaigning to oust Prime Minister Abhisit Vejjajiva, dissolve Parliament and hold new elections.
But it only served to anger the Red Shirts, who immediately declared a "war" on the government.
"From now on our mission is to hunt down Abhisit ... This is a war between the government and the Red Shirts," Arisman, a charismatic pop singer-turned-activist, told supporters after his escape.
Arisman is seen as a radical Red Shirt leader who has incited violence. He is wanted by police for leading an invasion of the Parliament building by hundreds of supporters on April 7 that forced lawmakers to climb a back wall to escape. VIPs were evacuated by helicopter. He had also led the storming of a Southeast Asian summit last year in the beach resort of Pattaya that forced the conference to be canceled.
On Friday, with a rope looped around his waist, a visibly nervous Arisman slid down from a third-story ledge of the hotel into a waiting crowd of cheering Red Shirt supporters who led him to a getaway car.
Arisman then returned and clambered on top of a van to give a short speech, to announce that the Red Shirts had seized two police officers - a colonel and a major general - as hostages to ensure his safety.
"I would like to thank all of the people who saved me - you have helped save democracy," said Arisman, a one-time crooner of love songs and a Thai heartthrob.
A second Red Shirt leader was seen climbing out of a hotel window and down a tree. It was not immediately clear if he escaped.
Thousands of Red Shirts, mostly rural poor, have congregated in Bangkok since March 12. They occupied two areas, one of which troops tried to clear on Saturday, leading to clashes that left 24 people dead and more than 800 injured in the worst political violence in nearly two decades.
The Red Shirts withdrew from that area Thursday and consolidated their forces at their second encampment in Rajprasong, the main shopping and hotel district of Bangkok.
A convoy of Red Shirt protesters escorted Arisman to Rajprasong from the hotel.
Earlier Friday, Deputy Prime Minister Suthep Thaugsuban announced on national television that a crackdown was being launched on the Red Shirts. He accused "terrorist elements" of infiltrating the Red Shirt organization to orchestrate Saturday's violence.
"The terrorists within the demonstrators used war weapons," Suthep said in a television message.
"I would like to ask innocent protesters to leave the demonstration area, in order to avoid being used as human shields," Suthep said. "The government from now on would like to carry out decisive legal measures against the Red Shirt leaders."