Syrian President Bashar al-Assad vowed to "cleanse" his country of extremism on Friday, a day after a suicide bomber killed more than 45 people including a key pro-regime Sunni cleric in a mosque.
The attack was also condemned by the opposition, who raised the possibility that the regime was behind the deadly blast at the Iman Mosque in Damascus on Thursday night.
State news agency SANA said that 49 people had been killed, including Mohamed Saeed al-Bouti, the most prominent Sunni cleric to back Assad's regime against a Sunni-led uprising.
The Syrian Observatory for Human Rights put the death toll at 46, saying that around 15 of the wounded were in critical condition and the toll was likely to rise.
The attack came as heavy fighting raged across Syria, and UN Secretary General Ban Ki-Moon said the United Nations would investigate whether chemical weapons have been used in the conflict.
In echoes of sectarian violence in Iraq, state media said the attack was carried out by a suicide bomber who blew himself up after entering the central Damascus mosque, where Bouti was addressing religious students.
His grandson was among the dead.
Bouti's death is a blow to Assad's regime, which has been fighting an insurgency that flared when his forces launched a bloody crackdown on Arab Spring-inspired democracy protests that erupted in March 2011.
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.
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.
In a statement issued by the presidency, Assad condemned the attack and mourned Bouti's death, vowing to eradicate "extremism and ignorance" in Syria.
"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," he said.
Ahmed Moaz al-Khatib, the head of the umbrella opposition National Coalition, condemned the cleric's killing as a "crime" and suggested the regime could have been behind it.
"We categorically condemn the assassination of the scholar Dr Mohamed Saeed Ramadan al-Bouti," Khatib told AFP in Cairo by phone.
"This is a crime, by any measure, that is completely rejected," he said. "Whoever did this was a criminal... and we suspect it was the regime."
Khatib, himself a cleric, said he had known Bouti, who was held in high esteem among Islamic theologians, but had disagreed with him over his vociferous support for Assad.
Born in 1929, Bouti was from a large Kurdish family and spent years studying Islam, including at Cairo's Al-Azhar University.
He was reviled by much of the opposition, and frequently lashed out against the rebels while encouraging Syrians to join the army to fight against them.
The regime is dominated by members of the minority Alawite sect and had relied in part on the cleric to bolster its claims to represent all Syrians, including the Sunni community, from which the opposition draws much of its support.
The attack came as rebels reportedly gained ground in the Golan Heights, which is partly occupied by Israel.
A Syrian security official told AFP some 2,500 trained and equipped rebels had entered the southern Daraa province in recent weeks, following reports that American experts were training rebels in the kingdom.
Louay Moqdad, a spokesman and coordinator for opposition forces, acknowledged some nations had begun training rebels, but declined to provide further details.
Observatory director Rami Abdel Rahman said more than 200 people had been killed throughout Syria on Thursday, including in the Damascus attack.