Sudanese police fired volleys of teargas at worshippers trying to leave a mosque to demonstrate after Friday prayers, a witness said, as the government tried to quell a protest movement angry about tough austerity measures.

The past three weeks have seen small-scale protests across Sudan calling for the government of President Omar Hassan al-Bashir, in power for 23 years, to resign.

Online activists, some of them inspired by the Arab Spring uprisings last year, have been using social media to call for even larger demonstrations. But Sudanese police and security forces have routinely and swiftly crushed any signs of dissent.

Hundreds of people have been arrested and detained and one journalist has been deported, Sudanese activists say.

On Friday, hundreds of Sudanese protesters left the Imam Abulrahman mosque in the Omdurman suburb of Khartoum en masse, only to be driven back inside by teargas.

"They had barely begun chanting for a minute. From the moment they left the mosque, the police fired teargas," one witness said. "They have now escaped inside and the police are surrounding the mosque's courtyard."

Police spokesman As-Sir Ahmed Omar said there had been a "limited protest which police contained without any losses".

The mosque, one of the country's largest and most famous, has been a frequent flashpoint for protests. It is associated with the opposition Umma party, which along with other opposition parties, backed demonstrations earlier this week, but has so far refrained from bringing out its supporters in large numbers.

At the Sayyid Ali mosque in the Khartoum suburb of Bahri, protesters were also forced back inside after police fired teargas as soon as they left to demonstrate.

Sudan has announced a raft of austerity measures to help keep the economy afloat after its oil revenues collapsed when South Sudan declared independence last year, taking three quarters of the oil output with it.

Protests that started on university campuses soon spread to other parts of the capital and beyond, but have rarely mustered more than a few hundred people.

The government of Sudan has dismissed the protesters as a few agitators acting on behalf of a Zionist-American plot.

The country witnessed popular uprisings in 1964 and 1985, which toppled ruling military rulers on both occasions.