A teacher, an engineer and a policeman are among at least 18 people dead after Myanmar police and military forces fired live bullets, stun grenades and teargas at hundreds of protesters.

Sunday has been deemed the bloodiest day of demonstrations since the February 1 military coup, which saw the army seize power and detain elected government leader Aung San Suu Kyi.

The United Nations Human Rights Office said it had received “credible information” about the use of deadly force against peaceful protesters in “several locations” in Myanmar.

It came after state television MRTV announced Myanmar’s UN ambassador Kyaw Moe Tun had been fired for having “abused the power and responsibilities of a permanent ambassador”.

He was accused of betraying the country after giving an impassioned plea at the UN General Assembly in which he urged the international community to use “any means necessary to take action” against the military.

“We need further strongest possible action from the international community to immediately end the military coup, to stop oppressing the innocent people, to return the state power to the people and to restore the democracy,” he said on Friday.

His speech was met with loud applause from many diplomats, with US ambassador Linda Thomas-Greenfield calling him “courageous”.

Mr Tun said he had been representing Ms Suu Kyi’s ousted government and held up three fingers, a Hunger Games symbol adopted by Myanmar protesters.

Myanmar’s state television announced on Saturday that Mr Tun had been stripped of his title, saying he had “spoken for an unofficial organisation which doesn’t represent the country”.

But the UN considers Mr Tun to be still in the post, with one anonymous official telling Reuters they do not officially recognise the junta as Myanmar’s new government.

People carry an injured man after police cracked down on demonstrators. Photo: AAP

Police were out in force and opened fire in different parts of the biggest city of Yangon on Sunday after stun grenades, tear gas and shots in the air failed to break up crowds. Soldiers also reinforced police.

Internet network engineer Nyi Nyi Aung Htet was among the first shot dead in Yangon.

The day before, he had posted on Facebook about the increasingly violent military crackdown on protesters.

“#How_Many_Dead_Bodies_UN_Need_To_Take_Action,” he wrote, in reference to the United Nations.

The UN Human Rights Office said at least 18 people were dead and more than 30 were wounded on Sunday, based on credible information it had received.

A wounded protester is carried during a protest against the military coup in Mandalay. Photo: AAP

Several wounded people were hauled away by fellow protesters, leaving bloody smears on pavements, media images showed. One man died after being brought to a hospital with a bullet in the chest, a doctor said.

“Myanmar is like a battlefield,” the Buddhist-majority nation’s first Catholic cardinal Charles Maung Bo said on Twitter.

A resident of the second city of Mandalay said one woman was shot in the head during the protests.

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

Police and the spokesman for the ruling military council did not respond to phone calls seeking comment on the protests.

The dead in Yangon included a teacher, Tin New Yee, who died after police swooped to disperse a teachers’ protest with stun grenades, her daughter and a fellow teacher said.

Security forces in Myanmar used lethal force as they intensified their efforts to break up protests. Photo: AAP

Police also hurled stun grenades outside a Yangon medical school, sending doctors and students in white lab coats scattering.

Junta leader General Min Aung Hlaing said last week authorities were using minimal force to deal with the protests.

But many protesters have now died in the turmoil. The army said a policeman has been killed.

The generals have traditionally shrugged off diplomatic pressure from other nations. They have promised to hold a new election but not set a date.

Suu Kyi, 75, who spent nearly 15 years under house arrest, faces charges of illegally importing six walkie-talkie radios and of violating a natural disaster law by breaching coronavirus protocols.

The next hearing in her case is on Monday.

-with AAP