Syrian government troops have killed more than 250 people in the city of Homs, opposition activists said.
They said shelling started on Friday and the attack was still ongoing on Saturday morning.
A video posted online purportedly shows victims being treated in a mosque which had been transformed into a makeshift hospital.
Euronews has been unable to independently verify its content because Syria restricts the movement of foreign journalists.
The official SANA news agency said foreign-backed terrorists executed the victims.
It accused international media of fabricating stories of a government crackdown.
President Bashar Assad’s response to the uprising has sparked uproar worldwide.
Anti-Assad demonstrators attacked the Syrian embassy in Berlin, Germany on Saturday.
Protesters also managed to storm the Syrian mission in Cairo. Similar rallies took place in Britain and the United States.
The developments come as the UN Security Council prepares to vote on a resolution on Syria.
Russia warns it will veto any call for regime change.
Syria is a long-standing Russian ally. The two countries enjoy close military and defence ties.