The secretary-general "calls for maximum restraint and urges for the safe release of all without violence or arrests," spokesman Stephane Dujarric said.

"Many of those trapped are women who were peacefully marching in commemoration of International Women's Day," he said.

According to the UN rights office "police began shooting and making arrests" in Yangon around 10:00 pm Monday, although it was unclear if they were arresting trapped protesters or newly arrived demonstrators.

Myanmar has been in turmoil since a February 1 coup ousted civilian leader Aung San Suu Kyi and triggered mass protests against the new military junta.

The police and military have responded with an increasingly brutal crackdown on demonstrators, with more than 50 people killed and nearly 1,800 arrested.

"We call on all to respect the rights of freedom of assembly and expression of the people of Myanmar as they demonstrate peacefully and express their hopes and desires for the future of their country," Dujarric said.

He also reported that the secretary-general had received a letter from a "special envoy" to the UN appointed by a committee representing Myanmar's parliament, which was forced underground following the coup.

"As far as I know we've not had any contact," with the individual, Dujarric said. "But we'll continue calling people who may have contact with him."

The envoy, who goes only by the name Dr Sasa, asked Guterres in the letter, which was obtained by AFP, to call on the Security Council "to help protect the people of Myanmar from further human rights violations."

He also asked for "robust, targeted sanctions -- not only on military leaders but on military enterprises and assets."

The missive, dated March 4, called for "a total arms embargo against the military, to ensure that more weapons do not get into areas already suffering military offensives."

Dujarric said that UN officials, including its special envoy on Myanmar, Christine Schraner Burgener, "have been in contact with elected parliamentarians as part of ongoing contacts with key stakeholders."