Activists say at least 35 people were killed in flashpoint cities like Hama, Deraa and Homs.
Eye witnesses say security forces clashed with tens of thousands of protesters in the suburbs of Damascus. Reports say the number of demonstrators is growing. Security forces were accused of firing live rounds, nail bombs and tear gas.
The UN says more than 5,000 people have been killed since the uprising against President Bashar al-Assad began in March. The Government says 2,000 security forces personnel have died.