YANGON (AFP) - Thousands of people joined a rally in Myanmar's main city on Saturday to call for changes to a military-drafted constitution that bars opposition leader Aung San Suu Kyi from becoming president.

The former political prisoner-turned-politician has been campaigning to amend the charter since she became a lawmaker two years ago.

The 2008 constitution blocks anyone whose spouse or children are overseas citizens from leading the country - a clause widely believed to be targeted at the Nobel laureate, whose two sons are British.

It also ring-fences a quarter of the seats in parliament for unelected military personnel, leaving the army with a significant political role despite the end of outright junta rule.

Addressing a crowd at the rally in Yangon, Ms Suu Kyi urged the military top brass as well as rank-and-file soldiers to support a petition campaign to amend the charter.

"I would like you all to consider whether getting more opportunities than ordinary citizens is really fair," Ms Suu Kyi said.

"The main strength of the military forces is weapons. So I would like you to consider whether getting special opportunities because of the power of arms is dignified or good for yourself," she said.

Parliamentary elections due to be held in 2015 are seen as a definitive test of whether the military is willing to loosen its grip on power.

The country's president is selected by the legislature, and Ms Suu Kyi has declared her ambition to lead the country.

Any change to the charter needs the support of over 75 per cent of the legislature, so at least some soldiers would have to vote for the reforms.

Ms Suu Kyi spent 15 years under house arrest during military rule in Myanmar, before she was freed after controversial elections in 2010 that her party boycotted.

Since then President Thein Sein has pushed through sweeping changes, including releasing other political prisoners and welcoming Suu Kyi and her party into parliament following landmark by-elections in 2012.