"The international community, including regional actors, must come together in solidarity with the people of Myanmar and their democratic aspirations," Christine Schraner Burgener said in a statement.

She said the Myanmar military was defying international calls for restraint, adding she had heard "heartbreaking accounts of killings, mistreatment of demonstrators and torture of prisoners" from contacts inside the Southeast Asian country.

"The ongoing brutality, including against medical personnel and destruction of public infrastructure, severely undermines any prospects for peace and stability," she said.

Myanmar's junta late Sunday imposed martial law in two densely populated Yangon townships.

More than 80 people have been killed in mass protests since the military wrenched civilian leader Aung San Suu Kyi from power -- a toll expected to rise dramatically after Sunday's violence.

The junta has repeatedly justified its power grab by alleging widespread electoral fraud in November's elections, which Suu Kyi's National League for Democracy party won by a landslide.

Soldiers and police have in recent weeks been staging near-daily crackdowns against demonstrators calling for a return to democracy -- deploying tear gas and firing rubber bullets and live rounds to quell anti-coup protests.