* Sunni cleric and dozens of worshippers killed in blast
* Buti supported Assad's battle against rebels
* Opposition leader condemns Buti's killing (Adds U.N. condemnation, updates death toll)
By Dominic Evans
BEIRUT, March 22 (Reuters) - President Bashar al-Assad vowed on Friday to purge Syria of "extremist forces" he accused of assassinating a leading Sunni Muslim cleric who backed his two-year battle against rebels and protesters.
Assad made the pledge in a message of condolence over the death of Mohammed al-Buti, who was killed along with dozens of worshippers by an explosion in a Damascus mosque on Thursday.
State media put the death toll from the blast at 49, but the Syrian Observatory for Human Rights which monitors violence across the country said 52 people died and the final figure was likely to be more than 60.
The mass killing inside a place of worship shocked many Syrians, long accustomed to the daily bloodshed of a conflict which has killed 70,000 people, displaced millions of others and devastated whole districts of Syria's ancient cities.
Authorities announced a day of mourning on Saturday, when a funeral is expected to be held for Buti, who often delivered his sermons in the historic Umayyad Mosque.
"Your blood ... and that of all Syrian martyrs will not be shed in vain," Assad said. "We will adhere to your thinking to eliminate their darkness and extremism until we purge our country of them."
The mosque bombing took place in the same Mazraa district of central Damascus where a car bomb killed more than 60 people one month ago, another sign that Syria's civil war had penetrated to the heart of Assad's capital.
Assad's artillery positions on the northern edge of Damascus pounded the rebel-held southwestern towns of Derayya and Moadamiya on Friday and a Damascus resident said the smell of gunpowder hung over the centre of the city.
The 47-year-old president has deployed air strikes, artillery barrages and Scud-type missiles to hit rebel fighters who control swathes of eastern and northern Syria and have challenged his hold over most of the country's main cities.
His government and the rebels accused each other of using a chemical weapon in clashes near the northern city of Aleppo on Tuesday in which 26 people were killed.
The United Nations has promised to investigate the incident, though a U.S. official has said it increasingly appeared a chemical weapon was not used.
U.N. CONDEMNS ATTACK
The United Nations Security Council "condemned in the strongest terms the terrorist attack in a mosque in Damascus", but added that any steps to combat terrorism must comply with international law on human rights and refugees.
Opposition leader Moaz Alkhatib, himself a former preacher at the Umayyad Mosque, said the killing of a Muslim scholar in a religious sanctuary was "a crime in every sense of the word".
"We could not agree with him politically, and believed he was wrong to stand with the rulers, but his killing opens up the gates to an evil that only God knows," he said in a statement.
Buti was a Sunni Muslim like most Syrians and the great majority of Syrian rebels, who were angered by his support for a president from the country's Alawite minority, an offshoot of Shi'ite Islam.
In one of his televised speeches, Buti described those fighting to topple Assad as 'scum'. The frail, 84-year-old preacher also used his position to call on Syrians to join the armed forces and help Assad defeat his rivals in the rebellion.
State television rebroadcast what it called Buti's last sermon at the Umayyad Mosque a week ago, in which he said Syria was under attack from the United States, Europe and al Qaeda, which he described as a Western creation.
But Alkhatib said there had been signs Buti was questioning his support for Assad and suggested he may have been killed by authorities "who feared that if he took a courageous decision it could overturn the whole balance (of power)".
Opposition activists have frequently blamed Syrian authorities in the immediate aftermath of major bombings in Damascus, many of which have subsequently been claimed by the Nusra Front - a rebel group which the United States has designated a terrorist organisation. (Additional reporting by Michelle Nichols; Editing by Andrew Heavens)