Security forces in Sudan's capital, Khartoum, have moved in to try to break up thousands of demonstrators calling for President Omar al-Bashir to resign.

Tear gas was fired from pick up trucks on the second night of a sit-in.

One protester told the BBC some in the military intervened to try to chase the security forces away, suggesting divisions among the armed forces.

Mr Bashir has so far refused demands to step down and make way for a transitional government.

One protester told the BBC's Newsday that a number of pick-up trucks arrived and began firing tear gas and live ammunition at the thousands of sit-in protesters.

She said the military was at first neutral but then tried to chase the security forces away.

It is unclear who the security forces were but BBC Africa editor Fergal Keane says reports indicate they were from the national intelligence service and a state militia.

The eyewitness said the security forces returned for a second attack and people then ran towards a navy facility to seek shelter from the prolonged firing.

Another eyewitness spoke of crowds forcing the attackers to flee.

There are unconfirmed reports of casualties from the scene.

One resident of a district 5km (3 miles) away told Reuters the tear gas could be felt there.

It started on Friday when protesters descended on the zone outside Sudan's army headquarters to call for Mr Bashir's removal.