A day-long clampdown by Myanmar's security forces killed 82 people in a town near Yangon, the Assistance Association for Political Prisoners monitoring group reported on Saturday.
Reports of the violence on Friday in Bago, 65 kilometers (40 miles) northeast of Yangon, took a whole day to emerge as the violence forced residents to flee to nearby villages.
The turmoil in Myanmar has intensified in recent weeks, with protesters refusing to submit despite the ongoing bloodshed following the coup that ousted civilian leader Aung San Suu Kyi in February.
Anti-coup protesters in Bago returned to the streets on Saturday, the Myanmar Now news outlet reported.
What happened in Bago?
Local media and activists reported that security forces fired rifle grenades at anti-coup protesters.
"It is like genocide," Myanmar Now quoted a protest organizer called Ye Htut as saying. "They are shooting at every shadow."
Security forces also refused to let rescue workers near the bodies, a resident told the French news agency AFP.
The United Nations office in Myanmar called for an immediate end to the violence on Saturday.
"The UN in Myanmar is following events in Bago with reports of heavy artillery being used against civilians and medical treatment being denied to those injured," the office announced on Twitter. "We call on the security forces to allow medical teams to treat the wounded."
What do we know about the total death toll?
The assault in Bago adds to current death toll of 618 civilians killed in Myanmar since the coup, according to the Assistance Association for Political Prisoners.
The military denies the death tolls reported by rights groups.
On Friday, junta spokesman Major General Zaw Min Tun said the military had recorded 248 civilian deaths and 16 police deaths. He added that security forces had not used automatic weapons against protesters.
Monitoring groups have faced difficulties in verifying data on the violence. The junta has suspended internet and mobile data services , pushing most of the population into an information blackout.
What happened on Saturday?
Anti-coup protesters also clashed with security forces on Saturday in the northwestern town of Tamu, near the Myanmar-India border, media reported.
Soldiers reportedly tried to take down barricades that protesters had set up. Two civilians were killed as soldiers started randomly shooting at protesters, a local told a news agency.
More than a dozen soldiers were killed when protesters retaliated and overturned a military truck by throwing a bomb, she said.
In the northeastern Shan state, an alliance of ethnic armies attacked a police station, killing at least 10 officers, according to media.
About a dozen armed groups in Myanmar have opposed the coup and pledged to stand with protesters as violence intensifies, prompting fears of a much deadlier civil war.