* At least 18 dead, UN says, as police crack down

* Protesters flee stun grenades

* Rights group condemns escalation of lethal force

* Hundreds detained in Saturday crackdown

Myanmar police fired on protesters around the country on Sunday 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.

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.

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 U.N. human rights office said.

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.

Among at least five killed in Yangon was internet network engineer Nyi Nyi Aung Htet Naing, who a day earlier had posted on Facebook about his concern at the growing crackdown, medics said.

Teacher Tin New Yee 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.

Three people were killed at Dawei in the south, politician Kyaw Min Htike told Reuters from the town. Two died in the second city of Mandalay, Myanmar Now media and a resident said. Resident Sai Tun told Reuters one woman was shot in the head.

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

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.

'OUTRAGEOUS'

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

Nevertheless, at least 21 protesters have now died in the turmoil. The army said a policeman had been killed.

The crackdown would appear to indicate determination by the military to impose its authority in the face of defiance, not just on the streets but more broadly in the civil service, municipal administration, the judiciary, the education and health sectors and the media.

'We are heartbroken to see the loss of so many lives in Myanmar. People should not face violence for expressing dissent against the military coup. Targeting of civilians is abhorrent,' the U.S. embassy said.

The Canadian Embassy said it was appalled. Indonesia, which has taken a diplomatic lead within the Association of Southeast Asian Nations (ASEAN) on the crisis, expressed deep concern.

Activists across Asia held rallies to support Myanmar's protesters in Myanmar with the rallying cry 'Milk Tea Alliance' which first united pro-democracy activists in Thailand and Hong Kong.

State-run MRTV television said more than 470 people had been arrested on Saturday. It was not clear how many were detained on Sunday.

Youth activist Esther Ze Naw said people were battling the fear they had lived with under military rule.

'It's obvious they're trying to instil fear in us by making us run and hide,' she said. 'We can't accept that.'

A day after the junta announced that Myanmar's U.N. envoy had been fired for opposing its rule by calling for action from the United Nations, the foreign ministry announced that diplomats at several other embassies were being recalled.

It gave no reason, but some diplomats have been among civil servants to join a Civil Disobedience Movement that has paralysed a swathe of official business.

While Western countries have condemned the coup and some have imposed limited sanctions, the generals have traditionally shrugged off diplomatic pressure. They have promised to hold a new election but not set a date.

Suu Kyi's party and supporters said the result of the November vote must be respected.

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.