At least 18 people were killed and more than 30 injured in Myanmar on Sunday as police and military forces "confronted peaceful demonstrations," the UN Human Rights Office said.
The reported casualties make Sunday the deadliest day since the military seized power on February 1.
As demonstrations against the military coup which ousted the democratically elected government of civilian leader Aung San Suu Kyi entered their fourth week on Saturday, security forces began a violent crackdown on protesters in towns and cities across the country.
The UN Human Rights Office said it had received "credible information" about the use of deadly force against peaceful protesters in "several locations" in Myanmar on Sunday.
"Deaths reportedly occurred as a result of live ammunition fired into crowds in Yangon, Dawei, Mandalay, Myeik, Bago and Pokokku," a statement from spokeswoman Ravina Shamdasani said. It also noted reports about the use of tear gas, flash-bang and stun grenades.
CNN has been unable to independently verify those claims.
The statement condemned the "escalating violence" and urged the military to "immediately halt the use of force against peaceful protesters," noting that "the people of Myanmar have the right to assemble peacefully and demand the restoration of democracy."
"These fundamental rights must be respected by the military and police, not met with violent and bloody repression," Shamdasani said.
The office also said police had detained at least 85 medical professionals and students, along with seven journalists during the demonstrations on Sunday, noting that more than 1,000 people had been "arbitrarily arrested and detained" in February, including "members of the democratically elected government."
The statement repeated the UN's call for the release of anyone detained arbitrarily. "The international community must stand in solidarity with the protesters and all those seeking a return to democracy in Myanmar," it said.
In a tweet, US Secretary of State Antony Blinken condemned violence in the country. "We condemn the Burmese security forces' abhorrent violence against the people of Burma & will continue to promote accountability for those responsible. We stand firmly with the courageous people of Burma & encourage all countries to speak with one voice in support of their will," he said.
Reuters earlier reported that Myanmar police had shot dead at least seven protesters Sunday and wounded several others, citing political and medical sources, as well as local media.
In the biggest city, Yangon, a protester was fatally shot when police opened fire on demonstrators, according to Reuters, citing a hospital doctor. The doctor, who asked not to be identified, said the protester was brought into hospital with a bullet wound in the chest. Local media outlet Mizzima also reported the death in Yangon's Thingangyun township.
Also in Yangon, a woman died of a suspected heart attack after police broke up a teachers' protest with stun grenades, according to Reuters who cited the woman's daughter and a colleague.
In the south of the country, three people were killed and more than a dozen were wounded when police opened fire on protesters in the town of Dawei, according to media outlet the Dawei Watch. Local politician Kyaw Min Htike confirmed police had shot protesters in Dawei.
Local media outlet Myanmar Now reported two people had been killed in a protest in the second-largest city of Mandalay, according to Reuters.