The army has cracked down on an uprising demanding a return to democracy since generals ousted and detained civilian leader Aung San Suu Kyi in February.

"We are receiving reports of scores killed, incl. children, 100s injured across 40 locations, & mass arrests," the UN Human Rights High Commissioner's office said in a Tweet.

"This violence is compounding the illegitimacy of the coup & the culpability of its leaders," it added.

Spokeswoman Ravina Shamdasani told AFP the UN body had not yet been able to independently confirm the reports, but added that multiple reports from at least 40 sites said that police and soldiers were responding to peaceful protests with deadly force.

She said the toll had risen to between 83 and 91 people killed along with hundreds of wounded.

Four reports said that children were among the victims, including at least one infant, the spokeswoman added.

The ruling junta in Myanmar staged Saturday a major show of might for its annual Armed Forces Day that included a huge military parade.

Violence then erupted all over the central Mandalay region as security forces opened fire on pro-democracy protesters.

UN officials and Western governments denounced the violence, with the US embassy in Myanmar blasting the junta led by General Min Aung Hlaing for "murdering unarmed civilians".

UK Foreign Secretary Dominic Raab issued a statement that said: "Today's killing of unarmed civilians, including children, marks a new low."