Myanmar security forces shot dead at least 18 protesters on Sunday, according to the U.N. human rights office, 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 yields power and releases ousted civilian leader Aung San Suu Kyi, who was detained along with top political allies at the start of the month.

Police and soldiers had already fired rubber bullets, tear gas and water cannons on demonstrations in recent weeks in an effort to bring the civil disobedience campaign to heel, with live rounds used in some isolated cases.

Large numbers were again mobilized on Sunday morning to scatter crowds in several parts of the country, after online calls for protesters to once again flood the streets.

Three men were killed and at least 20 others injured when security forces moved on a rally in the southern coastal hub of Dawei, said a volunteer medic and a media outlet based in the city.

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

"More wounded people keep coming in," he told Agence France-Presse (AFP).

Two teenagers were also 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.

The deaths were confirmed by media based in the town.

Officers in Yangon began dispersing small crowds minutes before the slated beginning of the day's protest, with one 23-year-old 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."

Local lawmaker Nyi Nyi who was ousted from his parliamentary seat by the coup confirmed the details of the death in a Facebook post.

A teacher, Tin New Yee, also lost her life after police swooped in to disperse a teachers' protest with stun grenades, sending the crowd fleeing, her daughter and a fellow teacher said.

Elsewhere in the city, 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.

The Myanmar Now media outlet reported two people had been killed in a protest in Mandalay, the second-largest city in Myanmar. Security forces fired again later in the day and one woman was killed, Mandalay resident Sai Tun told Reuters.

"The medical team checked her and confirmed she didn't make it. She was shot in the head," Sai Tun said.

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.

Weeks of unrest

Since the military takeover on Feb. 1, Myanmar has been roiled by giant demonstrations and a civil disobedience campaign encouraging civil servants to walk off the job.

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 (AP) photographer in Yangon.

Near the city's main university, police fired several stun grenades to clear a crowd at a protest hot spot.

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 coup has been fierce, with the United States, the European Union and other major powers denouncing violence against protesters.

'Anything can happen'

Suu Kyi has not been seen in public since she was taken into custody during predawn raids in the capital Naypyitaw as the coup was launched.

She is due to face court there on Monday under 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 the Nobel laureate ahead of the hearing.

"As a lawyer, I put my trust in the court and... a fair trial," 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.

The United Nations said there had been no official notice of the diplomat's firing and that he remained Myanmar's representative for the moment.

Before Sunday, at least five people had been killed since the army takeover – four of them from injuries sustained at anti-coup demonstrations.

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

Meanwhile, the U.K. said on Sunday that escalating violence against protesters in Myanmar was abhorrent and it called on the country's military leaders to restore democracy.

"Working with the U.S. and Canada, the UK has taken action by imposing human rights sanctions against nine Myanmar military officers, including the commander-in-chief, for their role in the coup," a spokesperson for Britain's Foreign Office said.