Myanmar activists have called for major anti-coup protests this weekend as the junta marks Armed Forces Day, after a firebomb attack on deposed civilian leader Aung San Suu Kyi's party headquarters.

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

Nearly 3,000 people have been arrested since the coup, according to a local monitoring group, but the junta earlier this week released more than 600 from Yangon's Insein prison.

On Friday a senior official from the jail, notorious for being the holding site of longtime political prisoners, said another 322 people have been freed.

"A total of 249 men and 73 women were released," he told AFP on condition of anonymity.

More than 300 people have died in anti-coup unrest in Myanmar, according to a local monitoring group
More than 300 people have died in anti-coup unrest in Myanmar, according to a local monitoring group AFP / STR

It comes on the eve of Saturday's Armed Forces Day, when the military will put on a show of strength with its annual parade.

Russian Defence Minister Alexander Fomin will be attending the event, and met with junta leader General Min Aung Hlaing on Friday, according to Russian news agency Interfax.

Fomin reportedly praised Myanmar as a "reliable ally" and "strategic partner" in the region, with a ministry statement saying that both sides will work to "strengthen military and technical cooperation".

Fears have been swirling that the day could become a flashpoint, as security forces continue to crack down on activists, protesters and political allies of Suu Kyi.

Before dawn on Friday, the Yangon offices of her National League for Democracy (NLD) were hit by a Molotov cocktail, which caused a brief fire.

The attack left only minor damage, but the party has been in disarray since the coup, with many of its top leaders including Suu Kyi in detention and some of its MPs in hiding.

Protesters out in force in Yangon on Friday
Protesters out in force in Yangon on Friday FACEBOOK / Handout

"We do not know who did this, but it is not good at all," Soe Win, an NLD member in charge of the headquarters, told AFP.

Activists issued a call for nationwide protests against the junta on Saturday.

"The time has arrived again to fight the military's oppression," prominent activist Ei Thinzar Maung posted on Facebook.

The protest movement has included widespread strikes and civil disobedience by government workers, which has hamstrung the functioning of the state.

In Mandalay, Myanmar's second largest city, doctors' white coats with black ribbons spray painted on them were hung at a clinic's entrance -- in apparent mourning for victims killed in the unrest
In Mandalay, Myanmar's second largest city, doctors' white coats with black ribbons spray painted on them were hung at a clinic's entrance -- in apparent mourning for victims killed in the unrest FACEBOOK / Handout

This has infuriated the authorities, who have used tear gas, rubber bullets and live rounds to break up street demonstrations, and arrested people suspected of supporting the civil disobedience campaign.

Security forces once again Friday deployed lethal arms in the southern city of Myeik against protesters wielding shields and homemade rifles, who moved quickly to carry the injured off the streets, according to AFP-verified footage.

At least three people were killed, including a woman who was in her house, said a resident near the crackdown, who witnessed the melee.

"They shot all the people along the road while they chased to arrest protesters on motorbikes," he told AFP, saying security forces were still indiscriminately shooting in the area.

Friday's killings will push the death toll since the coup to more than 320, according to The Assistance Association for Political Prisoners (AAPP), a local monitoring group.

To protect themselves from violence, some activists have come up with creative ways to protest, including staging "human-less" rallies, using objects or dummies in place of people.

In Mandalay, Myanmar's second-largest city, doctors' white coats were hung at a medical clinic's entrance -- in apparent mourning for those killed in the unrest.

US ambassador to Myanmar Thomas Vajda on Friday visited the site where 23-year-old Nyi Nyi Aung Htet was shot down last month and laid a wreath of white roses at the spot where he died.

The Karen National Union (KNU), a key ethnic rebel group, released a letter Friday addressed to junta leader General Min Aung Hlaing, acknowledging an invitation from the regime to meet.

"The KNU finds it completely unacceptable that police and (the military)... have killed, attacked, and threatened peaceful protesters," said the letter, dated March 22.

It called for the release of all those detained since the coup and for the military to "withdraw from active engagement in politics".

The KNU "can meet with him only after the Tatmadaw (military) implements the following essential wishes of the people," it said.

The group -- which has been fighting the military for decades for more autonomy -- is currently sheltering hundreds of anti-coup protesters who have fled to their militia-controlled territory in the country's east.

International players have heaped condemnation and sanctions on the generals.

The latest came overnight from the United States and former colonial power Britain, which has targeted a huge military-owned conglomerate that gives the army chiefs access to enormous wealth.