Syrian forces have opened fire on protesters who massed Friday to call once again for President Bashar al-Assad's resignation.

Rights groups and activists said security forces shot at demonstrators in areas that include the Damascus suburb of Doma, Dara'a province in the south and the eastern town of Deir Ezzor. They say at least two people were killed.

Meanwhile, Syria says it is investigating an attack Thursday on the country's best-known political cartoonist.

Ali Ferzat was kidnapped and severely beaten by assailants who then dumped him on the side of a road near Damascus. Activists say he was attacked by security forces. State-run media reports blame “veiled people” for the incident.

A relative of Ferzat told Western media the attackers threatened to break Ferzat's bones as a warning for him to stop aiming his cartoons against government officials.

Through his cartoons, Ferzat has become a noted critic of Syria's government and its almost six-month-old crackdown on dissent and pro-democracy demonstrators. Some of his drawings have mocked President Assad.

The United Nations says more than 2,000 people have died in Syria during the government crackdown. President Assad has blamed much of the deadly violence on what he calls armed “gangs” and “terrorists.”