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 on Twitter.

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

The UN office's spokeswoman Ravina Shamdasani told AFP: "We haven't yet been able to independently corroborate but we have received multiple credible reports from many locations across the country (at least 40 separate locations so far) of police and military units responding to peaceful demonstrations with lethal force.

"So far the death tolls reported range from 83-91 killed and hundreds injured. We have four reports of children being killed, including at least one infant."

The ruling junta in Myanmar staged Saturday a major show of might for the annual Armed Forces Day that included a huge military parade attended by delegations from China and Russia.

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".

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

Armed Forces Day commemorates resistance to Japanese occupation during World War II and traditionally involves a review of thousands of soldiers, along with tanks, missiles and helicopters.

During a speech at the parade, General Min Aung Hlaing issued another threat to the anti-coup movement that has gripped the country since he took charge, warning acts of "terrorism which can be harmful to state tranquillity and security" were unacceptable.