YANGON, March 1 (CNA) - Myanmar police fired on protests around the country on Sunday (Feb 28) in the bloodiest day of weeks of demonstrations against a military coup and at least 18 people were killed, the UN human rights office said.

Rescue workers told AFP that three men had been shot dead in southern Dawei city, while another two teenagers were killed in the town of Bago.

A sixth person died in Yangon, a lawmaker from Myanmar's ousted civilian government said in a Facebook post.

Myanmar has been in chaos since the army seized power and detained elected government leader Aung San Suu Kyi and much of her party leadership on Feb 1, alleging fraud in a November election her party won in a landslide.

The coup, which brought a halt to tentative steps towards democracy after nearly 50 years of military rule, has drawn hundreds of thousands onto the streets and the condemnation of Western countries.

"Myanmar is like a battlefield," the Buddhist-majority country's first Catholic cardinal, Charles Maung Bo, said on Twitter.

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

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, said a doctor who asked not to be identified.

"Police and military forces have confronted peaceful demonstrations, using lethal force and less-than-lethal force that – according to credible information received by the UN Human Rights Office – has left at least 18 people dead and over 30 wounded," the UN office said.

"Deaths reportedly occurred as a result of live ammunition fired into crowds in Yangon, Dawei, Mandalay, Myeik, Bago and Pokokku. Tear gas was also reportedly used in various locations as well as flash-bang and stun grenades."

Among the dead were three people in Dawei in the south, politician Kyaw Min Htike told Reuters from the town.

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

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

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

Police also hurled stun grenades outside a Yangon medical school, sending doctors and students in white lab coats scattering. A group called the Whitecoat Alliance of medics said more than 50 medical staff had been arrested.

Police broke up protests in other towns, including Lashio in the northeast, Myeik in the deep south and Hpa-An in the east, residents and media said.