The country has been in uproar since the military ousted the Nobel laureate in a lightning putsch on February 1, triggering an uprising demanding a return to democracy.

Her party, the National League for Democracy (NLD), has been in disarray since the coup, with some of its elected MPs in hiding.

At about 4 am Friday, an attacker hurled a Molotov cocktail at its Yangon HQ, causing a brief fire.

"When the residents nearby knew about the fire, they called the fire service department to put it out... it was under control by around 5 am," Soe Win, an NLD member in charge of the headquarters, told AFP.

"It seems that someone lit a Molotov cocktail and threw it at the headquarters."

Only the entrance of the office was scorched, and party members were already inside assessing the damage, he said.

"We have to file a complaint to the police... We do not know who did this, but it is not good at all," Soe Win said, declining to speculate the reason for the attack.

The incident comes on the eve of Armed Forces Day, when the military will put on a show of strength with its annual parade.

Fears have been swirling that the day could become a flashpoint.

The building was the site of many demonstrations in the first weeks after the coup, which the junta justified by citing fraud in November elections, which the NLD won.

But street rallies are now deadly affairs, as security forces increasingly deploy tear gas, rubber bullets and live rounds against protesters.

So far, 320 people have died in anti-coup unrest, according to a local monitoring group, though the junta announced a far lower toll on Tuesday of 164.