WRAPUP 7-Protests surge in Yangon as Myanmar forces trap youth protesters

(Recasts with protests, adds appeals from embassies, UN)

* Hundreds of youth protesters trapped by security forces

* Three dead, several injured in two towns - witnesses

* Shops, factories, banks closed in Yangon

* Over 50 people killed by security forces so far

March 8 (Reuters) - Thousands of people took to the streets of Myanmar's biggest city in defiance of a night curfew on Monday, chanting in anger after security forces besieged hundreds of young anti-coup protesters in one neighbourhood.

Western embassies appealed to the ruling military junta to allow the protesters to leave Sanchaung, where they were cornered at the end of another day of bloodshed in Myanmar in which at least three protesters were killed elsewhere in the country.

"Free the students in Sanchaung," people chanted in the streets in districts across the former capital, where daily protests have taken place for more than a month against the Feb. 1 coup which overthrew elected leader Aung San Suu Kyi.

In some areas, police used stun grenades and fired shots to try to disperse protesters, witnesses said.

Live video from Sanchaung on social media showed protesters running between the houses as stun grenades detonated.

"Almost 200 young protesters are still blocked by the police and soldiers there. Local and international community needs to help them now! Please," one protest leader, Maung Saungkha, said on Twitter.

The U.S. Embassy said in a statement: "We call on those security forces to withdraw and allow people to go home safely." The United Nations office in Myanmar and the British Embassy made a similar appeal.

In Geneva, the U.N. human rights office voiced deep concern about the fate of the protesters trapped in Sanchaung. Rights chief Michelle Bachelet said they should be allowed to leave safely and without reprisals.

A spokesman for the junta did not answer calls requesting comment.

Police said they would scrutinise family registration lists in the area to check for outsiders - threatening action against anyone caught concealing them.

State television MRTV said: "The government's patience has run out and while trying to minimise casualties in stopping riots, most people seek complete stability are calling for more effective measures against riots."

According to the United Nations, more than 50 people have been killed by security forces in the junta's attempts to end the protests demanding the release of Suu Kyi and other detainees and respect for the election she won last year.

AT LEAST THREE DEAD