The red-shirted protesters, who are calling for the resignation of Thai Prime Abhisit Vejjajiva, declared victory and walked away from the building.
"We have won. We have stopped them from holding a summit," Jakrapob Penkair, one of the protest leaders, said in the capital, Bangkok. "But we have not achieved our goal yet. We will continue to protest in Bangkok until Abhisit resigns."
The chaos is a huge embarrassment for Abhisit, who has been trying to project an image of calm and normality since taking power in a parliamentary vote four months ago after a court dissolved the previous government for election fraud.
It also raises tension in Thailand, where anti-government crowds as big as 100,000 marched in Bangkok this past week, and increased the threat of violence and a possible confrontation between the military and the protesters.
"The situation has gotten completely out of hand. Violence and bloodshed is very much possible," said Charnvit Kasetsiri, a historian and former rector of Bangkok's Thammasat University. "The country is very split and it might have reached the point of no return."
More than 1,000 demonstrators smashed through the convention center's glass doors and ran through the building, overturning tables, blowing horns, waving Thai flags and screaming, "Abhisit get out!"
They are supporters of former Prime Minister Thaksin Shinawatra, who was deposed in a 2006 military coup for alleged corruption and abuse of power. They say Abhisit was not elected by the people and should step down so new elections can be held.
The leaders were safely evacuated by helicopter from Pattaya to a nearby military airport and have left the country, said government spokesman Supachai Jaisamuth.
"The meeting cannot go on. We have to consider the security of the leaders," Supachai said. "The situation is too violent and it is a security concern for the leaders."
The protesters met little resistance from a thin line of unarmed soldiers in riot gear who were standing in front of the summit venue. When the protesters rushed toward the building and started pounding on the glass facade, the soldiers attempted to push them back but were overwhelmed by their numbers.
"We want to tell Abhisit himself that this meeting cannot go on," protest leader Arisman Pongreungrong said after leading the mob into the hilltop convention hall in Pattaya, a resort city about 140 kilometers (90 miles) southeast of Bangkok.
Nine leaders from Southeast Asian nations were in a nearby hotel on the convention grounds at the time the protesters broke in, said Foreign Ministry spokesman Tharit Charungvat. Abhisit said the leaders were "very understanding" about the abrupt cancellation.
