(Adds letter read to rights council in Geneva)

* Fresh demonstrations erupt across Myanmar

* Facebook restricts content run by Myanmar military

* Junta orders release of thousands of prisoners

* U.S. imposes sanctions on military leaders, businesses

Feb 12 (Reuters) - Supporters of ousted Myanmar leader AungSan Suu Kyi clashed with police on Friday as hundreds ofthousands joined nationwide pro-democracy demonstrations indefiance of the junta's call to halt mass gatherings.

The United Nations human rights office said more than 350people, including officials, activists and monks, have beenarrested in Myanmar since the Feb. 1 military coup, includingsome who face criminal charges on "dubious grounds".

The U.N. rights investigator for Myanmar told a specialsession of the Human Rights Council in Geneva that there were"growing reports, photographic evidence" that security forceshave used live ammunition against protesters, in violation ofinternational law.

Special Rapporteur Thomas Andrews urged the U.N. SecurityCouncil to consider imposing sanctions and arms embargoes.

Myint Thu, Myanmar’s ambassador to the U.N. in Geneva, toldthe session that Myanmar did not want "to stall the nascentdemocratic transition in the country," and would continueinternational cooperation.

Friday's mostly peaceful protests were the biggest so far,and came a day after Washington imposed sanctions on generalswho led the takeover.

Three people were wounded when police fired rubber bulletsto break up a crowd of tens of thousands in the southeasterncity of Mawlamyine, a Myanmar Red Cross official told Reuters.

Footage broadcast by Radio Free Asia showed police chargingat protesters, grabbing one and smashing him in the head. Stoneswere then thrown at police before the shots were fired.

"Three got shot – one woman in the womb, one man on hischeek and one man on his arm," said Myanmar Red Cross officialKyaw Myint, who witnessed the clash.

Several people in Mawlamyine were arrested but laterreleased when a thousands-strong crowd stood outside the policestation and demanded they be freed, according to live footagebroadcast by Radio Free Asia.

Doctors have said they do not expect a 19-year-old womanshot during a protest in the capital Naypyitaw on Tuesday tosurvive. She was hit in the head with a live round fired bypolice, witnesses said.

In the biggest city Yangon on Friday, hundreds of doctors inwhite duty coats and scrubs marched past the golden Shwedagonpagoda, the country's holiest Buddhist site, while in anotherpart of town, football fans wearing team kits marched withhumorous placards denouncing the military.

Other demonstrations took place in Naypyitaw, the coastaltown of Dawei, and in Myitkyina, the capital of northern Kachinstate, where young men played rap music and staged a dance-off.

Social media giant Facebook said it would cut thevisibility of content run by Myanmar’s military, saying they had"continued to spread misinformation" after seizing power.

The generals have sought to justify their takeover by sayingthere was fraud in an election last November won by Suu Kyi'sNational League for Democracy (NLD), a claim rejected by thecountry's election committee.

In a letter read out to the rights council in Geneva, some300 elected parliamentarians from Myanmar called on the UnitedNations to investigate "gross human rights violations" committedby the military since its coup.

CALL FOR 'MORE ACTIONS'

Supporters of the NLD welcomed the U.S. sanctions but saidtougher action was needed.

"We are hoping for more actions than this as we aresuffering every day and night of the military coup here inMyanmar, " Suu Kyi supporter Moe Thal, 29, told Reuters.

Myint Thu, Myanmar's ambassador to the U.N. in Geneva, toldthe special council session that his government wanted "betterunderstanding of the prevailing situation in the country, andconstructive engagement and cooperation from the internationalcommunity."

"We do not want to stall the nascent democratic transitionin the country," he said.

The coup has prompted the biggest demonstrations since a2007 "Saffron Revolution" that ultimately became a step towardthe democratic change that has now been halted.

Friday's protests marked the seventh consecutive day ofprotests, including one on Thursday outside the Chinese embassywhere NLD supporters accused Beijing of backing the junta,something China has denied.

Security forces carried out more arrests overnight Thursday.

PRISONER AMNESTY

The junta remitted the sentences of more than 23,000prisoners on Friday, saying the move was consistent with"establishing a new democratic state with peace, development anddiscipline" and would "please the public".

The Frontier Myanmar news magazine reported the prisonersgiven amnesty included four supporters of a gunman who shot deada prominent Suu Kyi ally and constitutional lawyer in 2017.

The protests have revived memories of almost half a centuryof direct army rule, punctuated by bloody crackdowns, until themilitary began relinquishing some power in 2011.

The generals have promised to stick to the 2008 constitutionand hand over power after elections, but on Friday the juntasaid it would "work for the emergence of a constitution that isin alignment and harmony with the Democratic Federal Union".

No date has yet been set for elections.

(Reporting by Reuters staffWriting by Matthew Tostevin, Stephen Coates, Raju Gopalakrishnanand Poppy McPhersonEditing by Lincoln Feast and Frances Kerry)