The leaders were being evacuated by helicopter from the seaside city of Pattaya to a nearby military airport, a security official said on condition of anonymity because he was not authorized to speak to the media.
The red-shirted protesters, who are calling for the resignation of Thai Prime Abhisit Vejjajiva, smashed through the glass doors of the convention hall and ran through the building, overturning tables, blowing horns, waving Thai flags and screaming, "Abhisit get out!
All the leaders were safe, said Foreign Ministry spokesman Tharit Charungvat. Nine leaders from Southeast Asian nations were in a nearby hotel on the convention grounds at the time the protesters broke in.
"The meeting cannot go on. We have to consider the security of the leaders," government spokesman Supachai Jaisamuth said. "The situation is too violent and it is a security concern for the leaders."
The chaos dealt a major blow to 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 scuttles a chance for the 16 regional leaders, including those from China, Japan and South Korea, to confer on ways to combat the global slump that has battered Asia's export-oriented economies. North Korea's recent rocket launch also was to be discussed at the weekend summit.
The protesters are supporters of former Prime Minister Thaksin Shinawatra, who was deposed in a 2006 military coup. They say Abhisit was not elected by the people and should step down so new elections can be held.
"We want to tell Abhisit himself that this meeting cannot go on," protest leader Arisman Pongreungrong said after leading the mob into the convention hall.
Abhisit, who has repeatedly refused to step down, went on national television and declared a state of emergency in the area surrounding the summit.
"I am using my power as the prime minister to declare a state of emergency in the area of Pattaya and Chonburi," he said. The decree gives the military the right to restore order, allows authorities to suspend civil liberties, bans public gatherings of more than five people and bars the media from reporting news that "causes panic."
Earlier, organizers said all morning meetings at the summit had been delayed after protesters blocked traffic around the hotels where delegates were staying and amid clashes between anti- and pro-government groups. There were unconfirmed reports of protesters carrying guns.
Government spokesman Panitan Wattanayagorn said two or three people were injured in the clashes and that authorities were investigating reports of shots fired and an explosion. Chinese Premier Wen Jiabao was scheduled to meet with the leaders of the 10-member Association of Southeast Asian Nations on Saturday morning. That was to be followed by meetings between ASEAN leaders and the president of South Korea, and later with Japan's prime minister.
A breakfast meeting of the foreign ministers from Japan, China and South Korea was also canceled.
After the protesters broke into the convention hall, a reporter saw at least three people with cuts from the glass and blood on the floor.
Panitan said protesters have been warned that "very tough standards will be applied to them very soon."