Supporters of ousted Myanmar leader Aung San Suu Kyi clashed with police on Friday as hundreds of thousands joined nationwide pro-democracy demonstrations in defiance of the junta’s call to halt mass gatherings.

The UN human rights office said more than 350 people, including officials, activists and monks, have been arrested in Myanmar since the February 1 military coup, including some who face criminal charges on “dubious grounds”.

The UN rights investigator for Myanmar told a special session of the Human Rights Council in Geneva that there were “growing reports, photographic evidence” that security forces have used live ammunition against protesters, in violation of international law.

Special rapporteur Thomas Andrews urged the UN Security Council to consider imposing sanctions and arms embargoes.

Myint Thu, Myanmar’s ambassador to the UN in Geneva, told the session that Myanmar did not want “to stall the nascent democratic transition in the country”, and would continue international cooperation.

Friday’s mostly peaceful protests were the biggest so far, and came a day after Washington imposed sanctions on generals who led the takeover. Three people were wounded when police fired rubber bullets to break up a crowd of tens of thousands in the southeastern city of Mawlamyine, a Myanmar Red Cross official told Reuters.

Footage broadcast by Radio Free Asia showed police charging at protesters, grabbing one and smashing him in the head.

Stones were then thrown at police before the shots were fired.

“Three got shot, one woman in the womb, one man on his cheek and one man on his arm,” said Myanmar Red Cross official Kyaw Myint, who witnessed the clash.

Several people in Mawlamyine were arrested but later released when a thousands-strong crowd stood outside the police station and demanded they be freed, according to live footage broadcast by Radio Free Asia.