Sudanese police fired tear gas on Friday at worshippers leaving a mosque in the metropolis chanting "freedom, freedom" in an anti-government protest after the main weekly Muslim prayers, witnesses said.

Worshippers have held regular demonstrations after prayers at the mosque run by the opposition Umma Party in the capital's twin city Omdurman since its veteran leader, former premier Sadiq al-Mahdi, threw his support behind the protest movement.

"Riot police swiftly moved in and fired tear gas at them," a witness told AFP.

"They were also chanting freedom, peace, justice," the rallying cry of the protests that erupted in December against the three-decade rule of President Omar al-Bashir, the witness said.

Demonstrations first broke out in the farming town of Atbara on December 19 after the government tripled the price of bread.

But they swiftly ecalated into a broader protest movement that has been seen as the biggest threat to Bashir's rule since he took power in an Islamist-backed coup that toppled Mahdi's elected government in 1989.

Officials say 30 people have died in violence related to the protests. Human rights groups say children and medics have been among more than 40 people killed.