A suicide bomb attack on a central Damascus mosque on Thursday killed 42 people, including Syria's most prominent pro-regime Sunni cleric, and wounded dozens of others, the health ministry said.
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.
The bomber blew himself up inside the Iman Mosque as the cleric, Mohamed al-Bouti, addressed religious students, in an attack that echoed sectarian violence in Iraq.
"The number of those martyred in the terrorist suicide attack in the Iman Mosque rises to 42 martyrs with 84 injured," state television said, citing the health ministry.
Bouti was the most senior pro-regime Sunni cleric in Syria whose weekly addresses at Friday prayers were frequently broadcast live on state television.
His death will be a blow to the regime of President Bashar al-Assad, 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 tens of thousands of people 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 among pools of blood.
Emergency workers collected the remains and carried them out in grey body bags.
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 be 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 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 Sunnis, from whom the opposition draws much of its support.
The attack came as fighting continued throughout the country, with rebels reportedly gaining ground in the Golan Heights, which is partly occupied by Israel.
"It appears that the rebels launched coordinated attacks on multiple parts of the Golan, taking control of areas and villages in the province of Quneitra," said Rami Abdel Rahman of the Syrian Observatory for Human Rights.
But state media reported the "armed forces restored security and stability to the towns of Al-Talal al-Hamr and Khan Arnaba (in Quneitra) after eliminating several Al-Nusra Front terrorists."
Lebanese security sources said about 220 Syrians, including some wounded, had crossed over from southern Syria in the past 24 hours.
A Syrian security official told AFP some 2,500 trained and equipped rebels had entered Daraa province in recent weeks, following reports American experts were training rebels in the kingdom.
Louay Moqdad, a spokesman and coordinator for opposition forces, acknowledged some nations had began training rebels, but declined to provide further details.
At least 120 people were killed in Syria on Thursday, according to an initial Observatory toll which was expected to rise sharply with the additional deaths in the mosque attack.