"Two people were killed in shelling by regime troops of the Raees mosque in the Waar neighbourhood" of Homs city, said the Syrian Observatory for Human Rights.
The group's director Rami Abdel Rahman said one of those killed was Sufwan Mashraqa, a Sunni Muslim cleric who was leading Friday prayers in the mosque.
Mashraqa was a former head of the city's department for religious affairs.
Amateur video distributed by activists showed the cleric giving a sermon. When the sound of shelling begins, scores of worshippers are overtaken by panic.
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It has become home to tens of thousands of people who have fled other parts of the city. Waar sees daily fighting and shelling.
Elsewhere in Syria, rebels battling to topple President Bashar al-Assad made a fresh advance in Aleppo, taking control of Kindi hospital in the north of the city, said the Observatory.
"We have confirmed reports that the Islamists and the (Al-Qaeda-affiliated) Al-Nusra Front have taken near-total control of the Kindi hospital, and that they killed at least 20 regular troops there," said Abdel Rahman.
The takeover came after "two Al-Nusra Front fighters detonated themselves at checkpoints guarding the facility", he told AFP.
Once a hospital, the facility was turned into a base for Assad loyalist troops several months ago.
Speaking to AFP via the Internet, Abu Omar said "it gives fighters in the north of the city easier access to the nearby countryside."
Shahba Press, a network of citizen journalists in Aleppo, said the takeover "comes a year into a (rebel) siege" of the hospital.
The advance comes days into what activists have described as an "unprecedented" bombing campaign on Aleppo's city and province.