Thirty-eight people have been killed in Myanmar as troops quelled protests in several towns and cities, the most violent day since demonstrations against last month’s military coup first broke out.

Police and soldiers opened fire with live rounds with little warning, witnesses say.

The bloodshed occurred a day after neighbouring countries called for restraint in the aftermath of the military’s overthrow of the elected government of Aung San Suu Kyi.

“It’s horrific, it’s a massacre. No words can describe the situation and our feelings,” youth activist Thinzar Shunlei Yi said via a messaging app.

The dead include four children, an aid agency said. Hundreds of protesters were arrested.

“Today it was the bloodiest day since the coup happened on the 1st of February. We had today – only today – 38 people died,” United Nations special envoy on Myanmar, Christine Schraner Burgener, said in New York.

“We have now more than over 50 people died since the coup started and many are wounded.”

A spokesman for the ruling military council did not answer calls seeking comment.

Schraner Burgener said in conversations with Myanmar’s deputy military chief Soe Win, she had warned him the military was likely to face strong measures from some countries and isolation in retaliation for the coup.

“The answer was: ‘We are used to sanctions and we survived’,” she told reporters.

“When I also warned they will go into isolation, the answer was: ‘We have to learn to walk with only few friends’.”

The UN Security Council is due to discuss the situation on Friday.

In Yangon witnesses said at least eight people were killed, seven when security forces opened sustained fire in a neighbourhood in the north of the city in the early evening.

“I heard so much continuous firing. I lay down on the ground, they shot a lot,” protester Kaung Pyae Sone Tun, 23, said.

In Washington, State Department spokesman Ned Price said the United States was “appalled” by the increase in violence.

The White House was evaluating an “appropriate” response and any actions would be targeted at Myanmar’s military, he added.

The United States has told China it is looking for Beijing to play a constructive role in Myanmar.

The European Union said the shootings of unarmed civilians and medical workers were clear breaches of international law.

It also said the military was stepping up repression of the media, with a growing number of journalists arrested and charged.

In the central town of Monywa, six people were killed, the Monywa Gazette reported.

Others were killed in the second-biggest city Mandalay, the northern town of Hpakant and the central town of Myingyan.

Save the Children said in a statement four children were among the dead, including a 14-year-old boy Radio Free Asia reported was fatally shot by a soldier on a passing convoy.

The soldiers loaded his body onto a truck and left the scene, according to the report.

Security forces breaking up protests in Yangon detained about 300 protesters, the Myanmar Now news agency reported.

Video posted on social media showed lines of young men, hands on heads, filing into army trucks as police and soldiers stood guard.

Images of a 19-year-old woman, one of two shot dead in Mandalay, showed her wearing a T-shirt that read “Everything will be OK”.

-AAP