Myanmar security forces opened fire on protestors Sunday, killing at least eight in the bloodiest action so far to smother opposition to the military coup four weeks ago.

The junta is battling to contain a massive street movement demanding it yield power and release ousted civilian leader Aung San Suu Kyi, who was detained along with top political allies at the start of the month.

Sunday brought a significant escalation in force, with fatal shootings of protesters in at least four cities around the country, as police and soldiers attempted to bring the civil disobedience campaign to heel.

The junta has ratcheted up its use of force over the weekend against the massive street movement demanding it yield power and release ousted civilian leader Aung San Suu Kyi The junta has ratcheted up its use of force over the weekend against the massive street movement demanding it yield power and release ousted civilian leader Aung San Suu Kyi Photo: AFP / Ye Aung THU

The United Nations condemned the deadly crackdown and said it had received credible information that as many as 18 people may have been killed.

"We strongly condemn the escalating violence against protests in Myanmar and call on the military to immediately halt the use of force against peaceful protesters," Ravina Shamdasani, spokeswoman for the UN human rights office, said.

AFP could not independently confirm the UN toll.

Police and soldiers had already fired rubber bullets, tear gas and water cannon on demonstrations over recent weeks Police and soldiers had already fired rubber bullets, tear gas and water cannon on demonstrations over recent weeks Photo: AFP / STR

Three men were killed and at least 20 others injured when security forces moved on a rally in the southern coastal hub of Dawei.

Rescue worker Pyae Zaw Hein said the trio were "shot dead with live rounds", while the injured were hit by rubber bullets.

"More wounded people keep coming in," he told AFP.

More than 850 people have been arrested, charged or sentenced since the coup, according to the Assistance Association for Political Prisoners More than 850 people have been arrested, charged or sentenced since the coup, according to the Assistance Association for Political Prisoners Photo: AFP / Sai Aung Main

Two teenagers were gunned down in Bago, a two-hour drive north of commercial capital Yangon.

Ambulance driver Than Lwin Oo told AFP he had sent the bodies of the 18-year-olds to the mortuary at Bago's main hospital.

Officers in Yangon began dispersing small crowds minutes before the slated beginning of the day's protest, with a 23-year-old man shot dead in the city's east.

"His wife is heartbroken," Win Ko, a social worker who visited the man's widow, told AFP. "She's three months pregnant."

International condemnation of the coup has been fierce International condemnation of the coup has been fierce Photo: AFP / Sai Aung Main

In Mandalay, an emergency doctor told AFP that two men had died from gunshot wounds.

Elsewhere protesters took up positions behind barricades and wielded homemade shields to defend themselves against the onslaught, with police using tear gas to clear some rallies.

Hundreds of people had been arrested by evening and transported to the city's notorious Insein Prison, where many of Myanmar's leading democracy campaigners have served long jail terms under previous dictatorships.

Police fired tear gas to disperse protesters in Tsuen Wan district Police fired tear gas to disperse protesters in Tsuen Wan district Photo: AFP / Philip FONG

At least one journalist documenting Sunday's assaults by security forces was beaten and detained further north in Myitkyina, a city at the headwaters of the Irrawaddy river, according to local outlet The 74 Media.

Another reporter was shot with rubber bullets while covering a protest in the central city of Pyay, their employer said.

A spokesman for the ruling junta did not respond to phone calls seeking comment on Sunday's violence.

Suu Kyi has not been seen in public since she was detained at the start of February Suu Kyi has not been seen in public since she was detained at the start of February Photo: AFP / Sai Aung Main

Before Sunday, at least five people had died in anti-coup unrest since the army takeover, including three killed on February 20.

One police officer also died while attempting to quell a protest, the military has said.

Myanmar has been roiled by giant demonstrations and a civil disobedience campaign encouraging civil servants to walk off the job since the February 1 military takeover.

Sunday's crackdown followed a similar wave of violent action against angry but largely peaceful anti-coup rallies around the country a day earlier.

Several journalists documenting Saturday's assaults by security forces were detained, including an Associated Press photographer in Yangon.

Human Rights Watch said medical volunteers helping to treat wounded protesters were also being targeted for arrest by security forces.

More than 850 people have been arrested, charged or sentenced since the coup, according to the Assistance Association for Political Prisoners monitoring group.

But the weekend crackdown was expected to raise that number dramatically, with state newspapers reporting 479 arrests on Saturday alone.

International condemnation of the military regime has been fierce, with the United States, European Union and others denouncing violence against protesters.

"We condemn the Burmese security forces' abhorrent violence against the people of Burma & will continue to promote accountability for those responsible," US Secretary of State Antony Blinken tweeted after Sunday's killings, using the country's old name.

"We stand firmly with the courageous people of Burma & encourage all countries to speak with one voice in support of their will," Blinken added.

Suu Kyi has not been seen in public since she was taken into custody during pre-dawn raids in the capital Naypyidaw.

A court hearing will be held on Monday for the ousted leader, who faces obscure charges for possession of unregistered walkie-talkies and violating coronavirus restrictions on public gatherings.

But her lawyer Khin Maung Zaw told AFP he had still been unable to meet with Suu Kyi ahead of the hearing.

"As a lawyer, I put my trust in the court," he said. "But in this period of time anything can happen."

State media announced Saturday that the junta had sacked the country's United Nations envoy, who gave an impassioned plea for help on behalf of Myanmar's ousted civilian government.