Burma's military rulers have put thousands of dissidents in jail
Leaders of 10 South East Asian nations meeting for a summit in Vietnam have been urged to put Burma's coming elections at the top of their agenda.
The agenda of the Association of South East Asian Nations (Asean) summit focuses on building regional ties.
But some 100 legislators in the region have appealed to the leaders to put pressure on Burma to ensure fair polls.
Thai PM Abhisist Vejajjiva will no longer attend, after declaring a state of emergency amid protests in Bangkok.
Officials had said Mr Abhisist would shuttle between Hanoi and Bangkok but correspondents say the situation is so delicate he now appears unwilling to leave the country.
The Asean summit is expected to focus largely on improving relations between the neighbours and regional trading partners.
Forming a free market group of 600 million people by 2015 is a key theme, although wide differences remain between the delegates.
The summit is focused on building stronger regional ties
A petition signed by more than 100 legislators in the region has appealed to the summit to take decisive action against Burma - a fellow member - with the aim of ensuring the elections are free and fair.
Burma's military junta has not yet given a date for the polls - the first in the country in 20 years - but they are expected to take place later this year.
Last month, the country implemented a series of electoral laws which effectively prevent the opposition from taking part in any meaningful way.
Some Asean members have criticised Burma's election plans and delegates of several countries have said they intend to raise the subject at the summit in Hanoi.
The Philippines and Indonesia have been outspoken in calling for a genuinely inclusive election in Burma, in which the opposition led by detained leader Aung San Suu Kyi, could take part.
Her party, the National League for Democracy (NLD), decided last week not to participate because of restrictions on campaigning, leadership and the continued detention of hundreds of its members.
Indonesian Foreign Minister Marty Natalegawa said the poll in Burma, also known as Myanmar, was "a potentially extremely important election".
"We've made references to the commitment by Myanmar that this will be an open, free, democratic and credible elections and we would like to see those kind of commitments realised."
Draft summit documents suggest the leaders will also be discussing how fast to cut back on economic stimulus measures, how to speed up construction of regional infrastructure and what action to take on climate change.
Host Hanoi has also spoken before the summit of its desire to secure a regional approach towards China's territorial claims in the South China Sea.
But the summit slogan, "From Vision to Action" is intended to refer to economic, not political progress.
"The building of the economic community will be one of the focal points during the summit," said Vietnam's Assistant Foreign Minister Pham Quang Vinh.
Asean combines the more developed economies of Indonesia, Singapore, Malaysia, Thailand, the Philippines, Vietnam and Brunei, with Burma, Laos and Cambodia.