YANGON (REUTERS) - Myanmar's major trade unions called on members to shut down the economy from Monday (March 8) to back a campaign against last month's coup, raising pressure on the junta as its forces fired weapons and occupied hospitals in the main city Yangon after a day of massive protests.

Witnesses reported sounds of gunfire or stun grenades in many districts of the commercial capital after nightfall, as soldiers set up camp in hospitals and university compounds, local media reported. It was not clear whether anyone was hurt.

The Assistance Association for Political Prisoners, a non-profit, said the army was "intentionally terrorising residents" in Yangon.

In a statement, nine labour organisations called on "all Myanmar people" to stop work in an effort to reverse the seizure of power by the military, which overthrew the civilian government on Feb 1.

"To continue economic and business activities as usual...will only benefit the military as they repress the energy of the Myanmar people," an alliance of nine unions said.

"The time to take action in defence of our democracy is now," they said in joint statement.

Mr Andrew Tillett-Saks, Myanmar Country Program Director and Regional Senior Organizing Specialist for the Solidarity Centre, a United States-based worker rights organisation, told Reuters many unions would begin a general strike immediately.

"But even more importantly it opens the door and increases the likelihood that many more from the private sector will answer the call in the days and weeks that follow... This is a strategy that could actually plausibly really pressure the military," he said.

A spokesman for the military junta did not answer phone calls seeking comment.

The call by the unions came as an official from the party of deposed leader Aung San Suu Kyi died overnight in police custody. The cause of death of National League for Democracy official Khin Maung Latt was not known.

Mr Ba Myo Thein, a member of the upper house of parliament which was dissolved after the coup, said reports of bruising to Mr Khin Maung Latt's head and body raised suspicions that he had been abused.

"It seems that he was arrested at night and tortured severely," he told Reuters. "This is totally unacceptable."

Police in Pabedan, the Yangon district where Mr Khin Maung Latt was arrested, declined to comment.

Some of the biggest protests in recent weeks were staged on Sunday. Police fired stun grenades and tear gas to break up a sit-in by tens of thousands of people in Mandalay, the Myanmar Now media group said. At least 70 people were arrested.

Police also launched tear gas and stun grenades in the direction of protesters in Yangon and in the town of Lashio in the northern Shan region, videos posted on Facebook showed.

A witness said police opened fire to break up a protest in the historic temple town of Bagan, and several residents said in social media posts that live bullets were used.

Video posted by Myanmar Now showed soldiers beating up men in Yangon, where at least three protests were held despite overnight raids by security forces on campaign leaders and opposition activists.

The United Nations says security forces have killed more than 50 people to stamp out daily demonstrations and strikes in the Southeast Asian nation since the military overthrew and detained Ms Suu Kyi on Feb 1.

"They are killing people just like killing birds and chickens," one protest leader said to the crowd in Dawei, a town in Myanmar's south. "What will we do if we don't revolt against them? We must revolt."

Reuters was unable to reach police for comment. A junta spokesman did not answer calls requesting comment.

Punched and kicked

The state-run Global New Light Of Myanmar newspaper quoted a police statement as saying security forces were dealing with the protests in accordance with law. It said the forces were using tear gas and stun grenades to break up rioting and protests that were blocking public roads.

Well over 1,700 people had been detained under the military junta by Saturday, according to figures from the Assistance Association for Political Prisoners advocacy group. It did not give a figure for overnight detentions.

"Detainees were punched and kicked with military boots, beaten with police batons and then dragged into police vehicles," AAPP said in a statement.

"Security forces entered residential areas and tried to arrest further protesters, and shot at the homes, destroying many." The killings have drawn anger in the West and been condemned by most democracies in Asia. The United States and some other Western countries have imposed limited sanctions on the junta.

China, Myanmar's giant neighbour to the northeast, said on Sunday it is prepared to engage with "all parties" to ease the crisis and is not taking sides.

"China is...willing to contact and communicate with all parties on the basis of respecting Myanmar's sovereignty and the will of the people, so as to play a constructive role in easing tensions," State Councillor Wang Yi, China's top diplomat, told a news conference.

Protesters demand the release of Ms Suu Kyi and respect for November's election - which her party won in a landslide but which the army rejected. The army has said it will hold democratic elections at an unspecified date.

Israeli-Canadian lobbyist Ari Ben-Menashe, hired by Myanmar's junta, told Reuters the generals are keen to leave politics and seek to improve relations with the United States and distance themselves from China.

He said Ms Suu Kyi had grown too close to China for the generals' liking.

Mr Ben-Menashe said he had also been tasked with seeking Arab support for a plan to repatriate Muslim Rohingya refugees, hundreds of thousands of whom were driven from Myanmar in 2017 in an army crackdown after rebel attacks.