A torrent of anger and defiance has brought hundreds of thousands of people out in nationwide rallies demanding the country's generals relinquish power.

Demonstrations have so far largely been peaceful, but police fired rubber bullets on students while dispersing a sit-down protest in the city of Mawlamyine, arresting at least nine.

The UN opened an emergency Human Rights Council with a warning that any international response to the coup should avoid harming vulnerable people, after the Biden administration unveiled targeted sanctions against senior regime figures.

With teachers, bureaucrats and air traffic controllers among the government employees walking off the job this week to demand an end to junta rule, the new military leader Min Aung Hlaing told striking workers to return to their offices.

"Those who are away from their duties are requested to return to their duties immediately for the interests of the country," he said, in a speech reported by state media.

Ahead of another day of mass rallies, state media announced the release of more than 23,000 inmates as part of a prison amnesty.

Friday was a public holiday in Myanmar and mass pardons to empty the country's overcrowded prison system are common on significant local dates.

More than 260 people have been detained in the days since the coup and about 20 released, according to the Assistance Association for Political Prisoners monitoring group.

There was no indication that Suu Kyi or any of her allies were among those set to be freed, and authorities have stepped up a crackdown on striking workers.

In the Irrawaddy Delta, home to much of Myanmar's rice crop, police stormed a medical clinic and detained a doctor who had been supporting the civil disobedience campaign as he was treating a patient.

"He was in the middle of putting stitches in his patient's head," the wife of Pyae Phyo Naing, 38, told AFP on Friday, a day after footage of the arrest went viral on social media.

News of the incident did not deter other medical workers from taking part in another day of massive rallies in commercial hub Yangon.

"Whatever pressure comes from the army chief, we will not pay attention," said Wai Yan Phyo, a doctor.

Further east in the port city of Mawlamyine, police moved quickly to clear a sit-in protest by university students.

Some of the demonstrators were briefly hospitalised after officers opened fire with rubber bullets.

But nine others taken into custody were later freed after a crowd mobbed a police station and demanded their release.

- Widespread dissent -

The coup has united disparate strands of society in opposition, with some reports of police officers breaking ranks to join demonstrations alongside celebrities, students and garment workers.

They have called for the junta to respect the results of November's elections, which saw Suu Kyi's National League for Democracy (NLD) party win in a massive landslide.

The military justified its takeover with claims of widespread voter fraud, though local and international monitors said there were no major issues that could have changed the outcome of the poll.

Min Aung Hlaing's regime has moved quickly to stack courts and political offices with loyalists after bringing the country's decade-old democracy to a sudden end.

Live rounds were fired at a rally in Naypyidaw this week, critically wounding two people -- including a woman who was shot in the head.

- 'Emergency' situation -

Concerns are growing over a draft cybersecurity bill that grants the regime power to order internet blackouts and website bans.

The junta has already ordered telecoms to block Twitter and Facebook and imposed a temporary nationwide internet shutdown last weekend.

Suu Kyi has not been seen since she was detained on February 1, the same day a new parliament was due to convene, but NLD officials have said the Nobel laureate was in "good health".

The military regime has weathered a chorus of condemnation from the US, European Union and former colonial power Britain since it took power.

"The international community will not accept the coup in Myanmar and we will hold those responsible to account," UK Foreign Secretary Dominic Raab said on Thursday.

But traditional allies of Myanmar's military, including Russia and China, slammed Friday's emergency Human Rights Council session as interference in "Myanmar's internal affairs".