Police in Sudan have "attacked" demonstrators with tear gas and rubber bullets, according to a rights group.

The protesters were leaving mosques for weekly Friday anti-regime regime protests that were ignited three weeks ago by rising inflation, when a witness said volleys of teargas were fired at them.

Security presence in neighbourhoods around Khartoum's centre has grown heavier in recent weeks.

"Still large numbers of police forces are surrounding the central mosques," said an official of the Organisation for Defence of Rights and Freedoms, representing political, media, trade union and other activists promoting human rights.

"It seems rubber bullets and tear gas were used," he said, adding that information was still preliminary.

The official with the rights group said there had been arrests.

Surrounding the mosque

One of the mosques targeted was that of the opposition Umma party in Khartoum's twin city of Omdurman, said the official, asking not to be named.

"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."

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.

Follow our in-depth coverage of anti-government demonstrations

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

Friday's demonstrations came two days after mainstream opposition political parties united ranks and officially announced the start of a peaceful campaign to overthrow the 23-year-old government of Omar al-Bashir.

The parties signed a political charter on Wednesday, which would govern an interim period in a post-Bashir Sudan.

Protests against high food prices began on June 16 at the University of Khartoum.

After Bashir announced austerity measures, including tax hikes and an end to cheap fuel, demonstrations spread to include a cross-section of people around the capital and in other parts of Sudan.

Friday has become the focus of the demonstrations, which initially occurred with groups of 100 or 200 people daily burning tyres, throwing stones and blocking roads in a call for regime change sparked by high inflation.

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