The latest chorus of international condemnation comes after the defence chiefs of the United States, Britain, Japan and nine other countries denounced Myanmar's military.
US President Joe Biden has led global condemnation of an "absolutely outrageous" crackdown by Myanmar's junta that left over 100 people, including several children, dead in the bloodiest day since the coup two months ago.
Soldiers and police have killed hundreds in brutal suppression against weeks of mass protests demanding a restoration of democracy and the release of detained civilian leader Aung San Suu Kyi.
At least 114 more people were killed on Saturday, the United Nations said, as the regime staged a major show of might for Armed Forces Day, an annual parade showcasing Myanmar's military prowess.
The death toll from crackdowns since the February 1 coup has climbed to at least 459, according to the Assistance Association for Political Prisoners (AAPP) monitoring group.
"It's absolutely outrageous and based on the reporting I've gotten, an awful lot of people have been killed totally unnecessarily," Biden told reporters on Sunday.
European Union foreign policy chief Josep Borrell said the junta's celebration of its armed forces had been blighted by "a day of horror and of shame".
"A professional military follows international standards for conduct and is responsible for protecting, not harming, the people it serves," the rare joint statement said after twelve countries' defence chiefs denounced the Myanmar military.
On Monday the British foreign ministry advised its nationals in Myanmar to leave as soon as possible, following "a significant increase in the level of recent violence".
Myanmar mourns the brutal murders of 114 anti-coup protesters by security forces. The use of lethal force by the junta has been condemned by defence ministers from a dozen countries pic.twitter.com/4EvwtefhPq— TRT World (@trtworld) March 28, 2021
AAPP said a further 13 people were killed on Sunday, as funerals were held for some of the victims, after the deadliest day of violence in the eight weeks since the coup.
In Mandalay, the country's cultural capital, the family of Aye Ko mourned at a funeral service after he was killed overnight.
"I am very saddened to lose my husband, together with my children I'm heartbroken," his wife Ma Khaing told AFP as she grieved with her four children.
'Shameful, cowardly, brutal'
Despite the dangers, protesters hit the streets again Sunday in parts of commercial hub Yangon and several other cities around Myanmar.
A 16-year-old boy lost his hand in one Yangon neighbourhood while trying to lob back a grenade that security forces had thrown at protesters, a rescue worker said.
The UN put Saturday's death toll at 107 people, including seven children, but expected it to rise further.
"The shameful, cowardly, brutal actions of the military and police, who have been filmed shooting at protesters as they flee, and who have not even spared young children, must be halted immediately," United Nations envoys Alice Wairimu Nderitu and Michelle Bachelet said in a joint statement.
Military-run broadcaster Myawaddy TV gave Saturday's death toll as 45 and said the crackdown was necessary because protesters had used guns and bombs against security forces.
'Harmful to state tranquillity'
An insurgent group in eastern Myanmar's Karen state said they had been targeted in air strikes late Saturday, hours after the ethnic armed group seized a military base.
Hsa Moo, an ethnic Karen and human rights activist, said three people were killed and at least eight wounded.
It was the first air assault in 20 years in the state, and targeted the Karen National Union (KNU), one of the country's largest non-state armed groups.
Further air strikes on Sunday sent 3,000 people fleeing through the jungle to seek safety across the border in Thailand, the Karen Women's Organization civil society group said.
Saturday's grand parade of troops and military vehicles in the capital Naypyidaw saw junta leader General Min Aung Hlaing defend the coup and pledge to yield power after new elections.
But he also issued a threat to the anti-coup movement, warning that acts of "terrorism which can be harmful to state tranquillity and security" were unacceptable.
Later that evening he entertained dignitaries including Russian deputy defence minister Alexander Fomin at a lavish outdoor dinner.