Unrest in Thailand intruded for the second straight year on a summit of Southeast Asian leaders as the Thai prime minister was forced to abandon his trip at the last minute Thursday amid fears of violence.

One day after declaring a national state of emergency, Prime Minister Abhisit Vejjajiva announced he would skip the 16th annual summit of the Association of Southeast Asian Nations in Vietnam to deal with thousands of "Red Shirt" anti-government protesters who have paralyzed parts of the Thai capital.

The tensions seized the attention of ASEAN leaders, who pride themselves on keeping one another's political affairs out of the limelight.

"The situation in Bangkok is worrying, and it's a somber backdrop to our discussions," Singaporean Foreign Minister George Yeo said at the summit's opening. "I really hope that the situation there will not lead to violence."

The Thai upheaval even upstaged concerns about Myanmar's upcoming elections, which have been boycotted by the main opposition party because its leader has been forbidden from participating.

Myanmar's junta plans to call elections sometime this year, but under the election laws, detained pro-democracy leader Aung San Suu Kyi is forbidden from participating.

Last week, members of her party, the National League for Democracy, announced they would not participate in the polls, the first in 20 years.

"It's disappointing that, because of the way the election laws have been crafted, it's not possible for the NLD to participate in the elections," Yeo said, speaking to reporters on the sidelines of the meeting Thursday morning.

However, he said, it was unlikely that the group would strongly criticize Myanmar's junta.

"We are not in a position to punish Myanmar," Yeo said. "If China and India remain engaged with Myanmar, then we have to."

Leaders from the 10 ASEAN nations had planned to focus on economics, where member countries with widely diverging political systems _ ranging from democracies to communism to a military junta _ share more common ground.

ASEAN hopes to advance its goals of forming a European-style economic community by 2015 and promoting development across the region.

Last year's summit was hijacked by the same protesters, forcing some visiting leaders to be airlifted out by helicopter. The Red Shirts, who briefly raided Parliament on Wednesday, are calling for Abhisit to dissolve Parliament within 15 days and call new elections. Instead, the prime minister has offered to do so by the end of the year. The Red Shirts claim he took office illegitimately in December 2008 with the help of military pressure on parliament.

The confrontation is part of the long-running battle between partisans of former leader Thaksin Shinawatra, ousted by a 2006 military coup, and those who oppose him. Thaksin was accused of corruption and showing disrespect to the country's revered monarch.

The demonstrators benefited from Thaksin's populist policies such as cheap health care and village loans.

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Associated Press writers Margie Mason and Tran Van Minh in Hanoi contributed to this report.