In his annual address to the UN Human Rights Council, Guterres mentioned only one country by name: Myanmar.

"Today, I call on the Myanmar military to stop the repression immediately," he said, speaking in a pre-recorded video message at the opening of the Geneva-based council's 46th session.

"Release the prisoners. End the violence. Respect human rights, and the will of the people expressed in recent elections," he said, insisting that "coups have no place in our modern world."

Myanmar's authorities have gradually ratcheted up their tactics against a massive and largely peaceful civil disobedience campaign demanding the return of ousted civilian leader Aung San Suu Kyi.

Saturday marked the deadliest day since the February 1 coup, with two people killed after security forces fired on a rally in Mandalay and a third man shot dead in Yangon.

And the junta warned protesters against inciting people "to a confrontation path where they will suffer the loss of life."

Protesters on Monday were undeterred by the warning, with tens of thousands rallying in Yangon, Myanmar's biggest city and commercial hub.

Guterres had already condemned Saturday's "deadly violence", insisting in a tweet Sunday that "the use of lethal force, intimidation and harassment against peaceful demonstrators is unacceptable."

Myanmar's foreign ministry hit back, accusing the UN and a number of foreign countries of "flagrant interference" in its internal affairs.

The United States, Britain and Canada have all responded to the coup with sanctions targeting Myanmar's top generals.

Even before the coup, army chief General Min Aung Hlaing -- who now holds legislative, executive and judicial powers -- was facing sanctions for his military's role in a brutal crackdown on the Rohingya Muslims in 2017.

European Union foreign ministers will meet Monday to discuss their own measures against the regime.

- 'Perfect storm' -

Since the army takeover, 640 people have been detained, according to the Assistance Association for Political Prisoners monitoring group.

Among those targeted have been railway workers, civil servants and bank staff, who have walked off their jobs as part of the anti-coup campaign.

"We see the undermining of democracy, the use of brutal force, arbitrary arrests, repression in all its manifestations. Restrictions of civic space," Guterres warned the council.

He also condemned attacks on civil society and "serious violations against minorities with no accountability, including what has rightly been called ethnic cleansing of the Rohingya population."

"The list goes on," he said.

"It is all coming together in a perfect storm of upheaval."

The UN chief voiced his "full support to the people of Myanmar in their pursuit of democracy, peace, human rights and the rule of law."

And he welcomed a resolution passed by the rights council earlier this month during a special session dedicated to the crisis in Myanmar, demanding the immediate release of Suu Kyi.

That resolution was adopted without a vote by the 47-member council, but several countries including traditional allies of Myanmar's military China and Russia disassociated themselves from the consensus.