(Reuters) - Myanmar police fired on protesters around the country on Sunday and at least 18 people were killed in the worst violence since a Feb. 1 military coup, the United Nations said, calling on the international community to act to stop the repression. Crowds of demonstrators came under fire in various parts of the biggest city of Yangon after stun grenades, tear gas and shots in the air failed to break up their protests. Across the country, protesters wearing plastic work helmets and with makeshift shields faced off against police and soldiers in battle gear, including some from units notorious for tough crackdowns on ethnic rebel groups in Myanmar’s border regions. “Severe action will be inevitably taken” against “riotous protesters”, the state-run Global New Light Of Myanmar said. The army had previously shown restraint, but could not ignore “anarchic mobs”. Several wounded people were hauled away in Yangon by fellow protesters, leaving bloody smears on pavements, media images showed. One man died after arriving at a hospital with a bullet in his 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, medics said. A day earlier he had asked on Facebook how many dead bodies it would take for the United Nations to take action. U.N. Secretary-General Antonio Guterres called on its members to do more. “The Secretary-General urges the international community to come together and send a clear signal to the military that it must respect the will of the people of Myanmar as expressed through the election and stop the repression,” U.N. spokesman Stephane Dujarric said. U.S. Secretary of State Antony Blinken condemned what he called “abhorrent violence” by Myanmar security forces. “We stand firmly with the courageous people of Burma & encourage all countries to speak with one voice in support of their will,” Blinken said on Twitter, adding the United States “will continue to promote accountability for those responsible.” 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. Outside a Yangon medical school, doctors and students in white lab coats scattered after police hurled stun grenades. 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. Nevertheless, at least 21 protesters have now died in the turmoil. The army said a policeman had been killed. Defiance of the coup has emerged not just on the streets but more broadly in the civil service, municipal administration, the judiciary, the education and health sectors and the media. Activists across Asia held protests in support, 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 were arrested on Saturday. It was not clear how many were detained on Sunday. “We are heartbroken to see the loss of so many lives in Myanmar,” 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. Youth activist Esther Ze Naw said people were battling the fear they had lived with under military rule. While some Western countries 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.