By Erika Solomon

BEIRUT, June 29 (Reuters) - Syrians in the besieged city of Douma wrapped mangled and bloodied corpses in white burial shrouds early on Friday, according to video posted online, after 190 people were killed in one of the deadliest days of Syria's 16-month-old uprising.

Activists said more than 50 of those killed on Thursday died in Douma, about 15 km (9 miles) outside the capital Damascus.

Video published on YouTube showed rows of shrouded bodies lining what activists said was a street in Douma. The Syrian Observatory for Human Rights said 41 people had died in the city, while other activists placed the toll at 59 or higher.

"Douma, the morning of June 29, 2012. This is the massacre committed against the people of Douma. God is our saviour. Two whole families are here (among the dead) ... God help us," said the man filming the scene.

One man held up the limp body of a girl, her pink blouse drenched in blood.

"This is another massacre of the massacres by Assad and his secret police," he said. "This is another massacre of the massacres by the international community, of all the great nations that have conspired against our people."

Douma has been under siege for weeks by security forces loyal to President Bashar al-Assad.

Activists say rockets have been raining down on the city for days amid heavy fighting between rebels and government forces. Video showed homes whose roofs had caved in and clouds of dust rising from crumbling buildings.

An activist called Mohammed Doumany told Reuters by Skype that 22 people from a single family had been killed.

"Dozens of the victims are still waiting to be buried, as cities continue to be under fire," said a statement from activists posted online. Many of the injured were in critical condition.

Syria's revolt has grown bloodier in recent weeks.

Rebels, apparently getting access to heavier weapons that can be used against tanks, have inflicted higher losses on Assad's forces.

The army has also intensified its onslaught, using helicopter gunships to attack rebels and laying siege to rebellious towns.

Opposition activists accuse the international community of inaction. Diplomacy has failed to produce an agreement between Western powers, who favour the opposition, and Russia, which has used its veto power at the United Nations Security Council to block Western and Sunni Arab moves to drive Assad from power. (Reporting by Erika Solomon and Mariam Karouny; Editing by Douglas Hamilton and Kevin Liffey)