US Secretary of State Antony Blinken has said that Washington is "horrified" by Saturday's deaths in Myanmar.
Dozens of people were killed by security forces during protests on the deadliest day since last month's military takeover of the country.
The killings, reportedly of more than 100 people, show "that the junta will sacrifice the lives of the people to serve the few," Mr Blinken said.
"The courageous people of Burma reject the military's reign of terror."
The US embassy previously said security forces were "murdering unarmed civilians", while the EU delegation to Myanmar said Saturday - officially Armed Forces Day - would "stay engraved as a day of terror and dishonour".
UN Secretary-General Antonio Guterres said he was "deeply shocked", and British Foreign Secretary Dominic Raab called it a "new low".
The lethal crackdown against civilians - including children - came as protesters defied warnings and took to the streets in towns and cities.
The Assistance Association for Political Prisoners (AAPP) monitoring group confirmed at least 91 deaths while local media put the figure higher.
"They are killing us like birds or chickens, even in our homes," resident Thu Ya Zaw told Reuters news agency in the central town of Myingyan.
"We will keep protesting regardless."
The latest violence took the number killed in the suppression of protests in Myanmar since the 1 February coup to more than 400.
The military seized control of the South East Asian country after an election which Aung San Suu Kyi's National League for Democracy (NLD) party won by a landslide.
What happened on Saturday?
Protesters gathered across Myanmar, also known as Burma.
State TV aired an announcement the previous evening saying people "should learn from the tragedy of earlier ugly deaths that you can be in danger of getting shot to the head and back".
Security forces were out in strength trying to prevent rallies.
Images shared on social media showed people with gunshot wounds and families mourning.
The director of the Burma Human Rights Network in UK told the BBC the military had shown it had "no limits, no principles".
"It's a massacre, it's not a crackdown anymore," Kyaw Win said.
Violent crackdowns using live ammunition were reported in more than 40 locations across the country.
Local news site Myanmar Now put the death toll at 114, while the United Nations said it was receiving reports of "scores killed" and hundreds more injured.
The AAP said among the fatalities was a 13-year-old girl who was shot dead inside her home.
Witnesses and sources told BBC Burmese of protester deaths in the cities and townships of Magway, Mogok, Kyaukpadaung and Mayangone.
Deaths were also reported in Yangon and on the streets of the second-largest city Mandalay, where protesters carried the flag of the NLD and gave their now traditional anti-authoritarian three-finger salute.
The military has not commented on the killings. In an Armed Forces Day TV address, coup leader Min Aung Hlaing said the army wanted to "join hands with the entire nation to safeguard democracy".
"Violent acts that affect stability and security in order to make demands are inappropriate," he said.
Meanwhile, an ethnic armed group in eastern Myanmar said military jets had targeted territory it controls. The strike was launched hours after the group, the Karen National Union, said it had overrun an army post near the Thai border.
It came amid rising tensions between the group and the military after years of relative peace.
Children among the dead and injured
Moe Myint, BBC Burmese
Across the country, children are amongst the injured and the dead in the bloodiest day since the coup on the 1 February.
Fourteen-year-old Pan Ei Phyu's mother says she rushed to close all the doors when she heard the military coming down her street. But she wasn't fast enough. A moment later, she was holding her daughter's blood-soaked body.
"I saw her collapse and initially thought she just slipped and fell. But then blood spurted out from her chest," she told BBC Burmese from Meiktila in central Myanmar.
It was the randomness of today's killings that was particularly shocking. Armed with battlefield weapons, the security forces appeared willing to shoot anyone they saw on the streets. The brutality they showed they were capable of is on another level from what we have seen since the coup.
Neither side - the military nor the pro-democracy movement - is willing to back down. The military think they can terrorise people to achieve "stability and security". But the movement on the streets, led by young people, is determined to rid the country of the military dictatorship once and for all.
It's painful to have to count the mounting dead, especially the children.