WRAPUP 10-Military tightens grip, death toll among anti-coup protesters rises as Myanmar seethes

* 20 killed in protests day after dozens shot dead - activists

* US, UN decry continuing violence

* Martial law courts given wide powers

* Suu Kyi court appearance postponed

March 15 (Reuters) - Myanmar security forces shot dead at least 20 pro-democracy protesters on Monday, an activist group said, and the military junta imposed martial law in parts of the main city Yangon, giving commanders wide powers to stamp out dissent.

Supporters of detained elected leader Aung San Suu Kyi took to the streets again despite the killing of dozens of protesters on Sunday in the bloodiest day since a military coup on Feb. 1 ignited mass demonstrations nationwide.

Marches took place on Monday in the second city Mandalay and in the central towns of Myingyan and Aunglan, where police opened fire, witnesses and media reported.

"One girl got shot in the head and a boy got shot in the face," an 18-year-old protester in Myingyan told Reuters by telephone. "I'm now hiding."

The Assistance Association for Political Prisoners (AAPP) said at least 20 people had been killed on Monday in shootings by security forces. The Myanmar Now media outlet reported protests across the country, including in the Yangon district of Hlaingthaya, the scene of unrest and arson attacks the previous day.

A journalist in Mandalay said one person was shot dead there after a big protest had passed off peacefully.

A junta spokesman did not answer calls requesting comment and Reuters could not independently confirm all the casualties.

The AAPP said 74 people died on Sunday, many of them in demonstrations in Hlaingthaya, a factory area.

In total, 183 people have been killed by security forces in the weeks of protests against the coup and the casualties were drastically increasing, the group said.

In Washington, the U.S. State Department said the military's violence against protesters was "immoral and indefensible".

United Nations Secretary-General Antonio Guterres urged the Myanmar military to allow a visit by his special envoy to help calm the situation and set the stage for dialogue and a return to democracy.

"The killing of demonstrators, arbitrary arrests and the reported torture of prisoners violate fundamental human rights and stand in clear defiance of calls by the Security Council for restraint, dialogue and a return to Myanmar's democratic path," his spokesman, Stephane Dujarric, said.

MARTIAL LAW

Myanmar state broadcaster MRTV said martial law had been imposed in several districts of Yangon, the country's commercial hub and former capital. Myanmar Now said it had also been imposed in several parts of Mandalay.