Syrian President Bashar al-Assad has vowed to "cleanse" his country of extremism, a day after a blast in a central Damascus mosque killed scores of people, including a key pro-regime Sunni cleric.

State news agency SANA said that 49 people died along with Mohamed Saeed al-Bouti, the most prominent cleric to back Assad's regime in his fight against opponents.

In a statement issued by the presidency, Assad condemned the attack and mourned Bouti's death, vowing to eradicate "extremism and ignorance" in Syria.

The attack was condemned by the opposition, who raised the possibility that the regime was behind the deadly blast at the Iman Mosque on Thursday night.

State media said the attack was carried out by a suicide bomber who blew himself up after entering the mosque, where Bouti was addressing religious students.

State media said Bouti's grandson was among the dead.



'Destroying extremism'

"I swear to the Syrian people that your blood, and that of your grandson and all the martyrs of the homeland, will not be spilled in vain because we will be faithful to your ideas by destroying their extremism and ignorance until we have cleansed the country," Assad said in the statement on Friday.

Pro-government television aired gruesome footage from inside the mosque, where dozens of corpses and body parts, including limbs and hands, were strewn on the carpeted floor in pools of blood.

The government declared Saturday as a day of mourning and state-run Syrian TV halted its regular programmes on Friday to air readings from the Muslim holy book, the Quran, as well as speeches of the late cleric.

Al-Bouti was the most senior religious figure to be killed in Syria's civil war and his slaying was a major blow to Assad.

The preacher had been a vocal supporter of the regime since the early days of Assad's father and predecessor, the late President Hafez Assad, providing a Sunni cover and legitimacy to their rule.

Sunnis are the majority sect in Syria while Assad is from the minority Alawite sect, an offshoot of Shia Islam.

In a speech earlier this month, al-Bouti had said it was “a religious duty to protect the values, the land and the nation'' of Syria.



Chemical weapons

"There is no difference between the army and the rest of the nation," he said at the time - a clear endorsement of Assad's forces in their effort to crush the rebels.

The latest attack came as heavy fighting raged across Syria.



Ban Ki-Moon, the UN chief, said the United Nations would investigate whether chemical weapons have been used in the conflict.

The relentless violence has killed more than 70,000 people, according to UN figures, and caused more than one million to flee their homes as refugees.

The mosque bombing was among the most serious security breaches in Damascus.



Last July, an attack that targeted a high-level government crisis meeting killed four top regime officials, including Assad's brother-in-law and the defense minister.

Last month, a car bomb that struck in the same area, which houses the headquarters of Syria's ruling Baath party, killed at least 53 people and wounded more than 200.