"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 human rights office said.
Speaking at the UN General Assembly on Friday, Kyaw Moe Tun urged the global community to use "any means necessary to take action" against the military to help "restore the democracy", saying he was representing Ms Suu Kyi's ousted government.
State-run television quoted security authorities as saying they detained 479 people on Saturday.
Security forces also tried to thwart protests in Mandalay, where roadblocks were set up at several key intersections and the regular venues for rallies were flooded with police. The journalist, Thein Zaw, remains in police custody.
Suu Kyi has now been detained alongside other government leaders including President Win Myint. The Foreign Correspondents' Club of Myanmar also condemned the arrest.
The Democratic Voice of Burma reported that as of 5 p.m.in Myanmar, there had been 19 confirmed deaths in nine cities, with another 10 deaths unconfirmed.
On Saturday, protesters occupied roads in the largest city, Yangon, and elsewhere. The reports from Monywa also said dozens or more people were arrested.
Confirming the deaths of protesters has been hard amid the chaos and general lack of news from official sources, especially in areas outside Yangon, Mandalay and Naypyitaw, the capital.
More than 770 people have been arrested, charged and sentenced since the February 1 putsch, according to the Assistance Association for Political Prisoners monitoring group, with some 680 still behind bars. Suu Kyi was arrested in the coup and faces a possible prison sentence on charges of illegally importing walkie-talkies and violating Covid-19 restrictions while campaigning a year ago.
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Reports on social media identified by name one young man believed to have been killed in Yangon. His body was shown in photos and videos lying on a sidewalk until other protesters carried him away.
Security forces opened fire on unarmed demonstrators in four cities on Sunday, with the United Nations saying it had credible information at least 18 people had died. Photos on social media showed one wounded man in the care of medical personnel.
AFP independently confirmed eight deaths in Sunday's violence, although there were fears the toll could be much higher. "And we condemn the security forces' brutal killing of unarmed people".
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US officials including Secretary of State Antony Blinken also condemned the violence.
"We call on the Myanmar military authorities to exercise utmost restraint, desist from the use of lethal force, and take immediate steps to de-escalate the situation to prevent further violence, bloodshed, and deaths", added Dr Balakrishnan.
Washington has imposed sanctions on Myanmar because of the coup, and Mr Sullivan said it would "impose further costs on those responsible", promising details "in the coming days".
The coup, which brought a halt to tentative steps towards democracy after almost 50 years of military rule, has drawn hundreds of thousands onto the streets and the condemnation of Western countries.
Min Aung Hlaing seized power after detaining Suu Kyi and other senior members of her party, the National League for Democracy, alleging fraud in November's election that gave the NLD a landslide victory.
Sunday's violence erupted in the early morning when medical students were marching in Yangon's streets near the Hledan Centre junction, which has become the gathering point for the protesters who then fan out to other parts of the city.
They scattered into smaller residential streets and started building makeshift barricades out of barbed wire and tables to stop the police.
There were more protesters on the streets throughout Sunday despite the presence of trigger-happy security forces.
"The Myanmar security forces' clear escalation in use of lethal force in multiple towns and cities ... is outrageous and unacceptable", Phil Robertson, deputy Asia director of New York-based Human Rights Watch, said in a statement.
Officers in the commercial hub Yangon began dispersing one crowd in the downtown area minutes before the slated beginning of the day's protest, but it was unclear whether they used live rounds.
The country has been rocked by protests since top government leaders, including Aung San Suu Kyi, were overthrown and detained after the army took power on 1 February.
Police in Dawei, in the southeast, and Monywa, 135 kilometres (85 miles) northwest of Mandalay, used force against protesters.