YANGON (REUTERS) - Myanmar anti-coup activists launched protests on Wednesday (April 21) calling for the release of prisoners detained by the junta, as the military government declared a National Unity Government formed by its opponents was illegal.

The South-east Asian country has been in crisis since the military seized power from Ms Aung San Suu Kyi's elected government on Feb 1, with almost daily protests against the coup occurring despite a brutal crackdown by the junta on dissent.

The Assistance Association for Political Prisoners (AAPP), an activist group, says 738 people have been killed by security forces since the coup and 3,300 people are currently in detention, including 20 who have been sentenced to death.

"Please raise your voice and demand the release of all the people who are being unfairly detained under the junta government," protest leader Ei Thinzar Maung said on Facebook.

On Wednesday, people shared photos on social media of themselves wearing blue shirts and holding up a hand with the name of an arrested person written on it.

The shirts are a tribute to pro-democracy activist Win Tin, who was imprisoned by the military for 19 years and died on April 21, 2014.

After his release, he pledged to wear a blue shirt until all political prisoners were freed.

The junta has released from jail thousands of people since the coup but relatively few have been linked to the protests.

Meanwhile, military-backed television broadcaster MWD reported on Tuesday that the Home Affairs Ministry had declared unlawful the National Unity Government (NUG) formed by opponents of the junta.

Pro-democracy politicians including ousted members of Parliament on Friday announced the formation of the NUG that nominally includes Ms Suu Kyi, who has been in detention since the coup, as well as leaders of the protests and ethnic minorities.

The NUG says it is the legitimate authority and has requested international recognition and an invitation to a meeting in Jakarta on Saturday to discuss the Myanmar crisis hosted by the Association of South-east Asian Nations (Asean).

Human Rights Watch said in a statement that the 10-member bloc, which includes Myanmar, should immediately withdraw an invitation to junta leader Min Aung Hlaing.

"Min Aung Hlaing, who faces international sanctions for his role in military atrocities and the brutal crackdown on pro-democracy protesters, should not be welcomed at an intergovernmental gathering to address a crisis he created," said Human Rights Watch Asia director Brad Adams.

Asean has been trying to guide Myanmar, out of the bloody turmoil triggered by the coup, but the group's principles of consensus and non-interference have restricted its ability to overcome members' divergent views on how to respond to the army's killing of hundreds of civilians.

Brunei Sultan Hassanal Bolkiah, who is also prime minister, will chair Saturday's meeting, his office said in a statement.

Myanmar's military has shown little willingness to engage with its neighbours and no sign of wanting to talk to members of the government it ousted, accusing some of them of treason, which is punishable by death.

UN Secretary-General Antonio Guterres' special envoy on Myanmar, Ms Christine Schraner Burgener, has communicated with the military since the coup, but the junta has not allowed her to visit.

Malaysia's Foreign Minister Hishammuddin Hussein said Prime Minister Muhyiddin Yassin would attend Saturday's meeting.

"We hope that with the coming discussions in Jakarta, Myanmar will agree to accept representatives from the Asean chair Brunei or the Asean secretariat in Jakarta to observe and help Myanmar return to normalcy," he told reporters.

However, Thai Prime Minister Prayut Chan-o-cha said he would be represented by Deputy Prime Minister Don Pramudwinai, who is also Foreign Minister.

Thai and Malaysian officials have said that General Min Aung Hlaing would go to Jakarta, although the Myanmar government has not commented.