Thousands of people have rallied in Serbia's capital for 11th week in a row against populist President Aleksandar Vucic and his government.

The crowds at Saturday's protest lit flares outside the parliament building chanting anti-government slogans. Opposition parties meanwhile offered an "agreement" to the citizens pledging to restore democracy they say Vucic has stifled.

The street protests have spread to other towns challenging Vucic, a former extreme nationalist who now says he supports Serbia's European Union membership bid.

Critics accuse Vucic of stifling hard-won democratic freedoms in the country that went through a series of wars during the 1990s. Protests against Vucic started after thugs beat up an opposition leader last November.

Beleaguered opposition parties say they will boycott any future elections in Serbia unless they are free and fair.