"I have completed my task and have ordered the lifting of the state of emergency," he said in a brief televised statement on Saturday.
Some of the Asean leaders were evacuated from Pattaya by helicopter, security officials said.
Thani Thongphakdi, the deputy spokesman from the Thai ministry of foreign affairs, told Al Jazeera on Saturday: "Leaders have expressed their understanding and it was agreed that the meeting would effectively be postponed."
The red-shirted protesters, supporters of Thaksin Shinawatra, declared victory thereafter and withdrew peacefully from the summit venue.
Sean Boonpracong, a spokesman for Democracy Against Dictatorship (UDD), the group leading the anti-government protests, told Al Jazeera: "We are satisfied that we are showing ourselves in force, to show that all is not well politically in Thailand.
"We feel like the larger issues in society have not been addressed at all. There is no justice in Thailand today."
Rival protesters - both monarchists and pro-Thaksin supporters - clashed among themselves, throwing stones and smoke bombs at each other about five kilometres from the venue.
A Thai official said two or three people were injured in the clashes. Authorities were investigating reports of shots fired and an explosion.
Leaders from the 10 Asean members plus China, Japan, South Korea, India, Australia and New Zealand were to discuss measures to combat the global economic crisis on Saturday and Sunday.
China and the Asean states were also due to sign a long-planned free-trade agreement, a key in the regional body's ambition to create the world's largest free-trade area.
But the protests overshadowed the summit and humiliated the Thai authorities, Al Jazeera's Tony Cheng, reporting from Pattaya, said.
|Red-shirted protesters say Abhisit's government is illegitimate [AFP]
"It's a day that has seen complete mayhem here," he said.
"It's humiliating for the Thai government who quite clearly had lost control of the situation.
"They have been caught by surprise by the number and persistence of the red-shirts."
The government had deployed 8,000 police in and around the summit in an attempt to control the protests, and Abhisit had pledged earlier that it would go ahead without a hitch.
The protesters had argued that the government was not empowered to sign any summit agreements, because the prime minister came to power in December through parliamentary defections they say were engineered by the army.