A Darfur rebel commander said on Saturday his JEM group had entered Khartoum and was aiming to take power in Sudan.
Khartoum was placed under an overnight curfew after fighting in the west of the capital.
Heavy gunfire was heard and helicopters and army vehicles headed towards the suburb of Omdurman, witnesses said. Artillery was heard later, becoming more frequent as the evening wore on.
The Darfur Justice and Equality Movement (JEM) rebels said they had taken control of Omdurman which lies on the opposite bank of the River Nile from Khartoum.
"We are now trying to control Khartoum. God willing we will take power, it's just a matter of time," senior JEM commander Abdel Aziz El Nur Ashr told Reuters by telephone.
"We have support from inside Khartoum even from within the armed forces." It would be the first time a rebel group has entered Khartoum.
Darfur rebels fought battles with Sudan's army in the North Kordofan province bordering Khartoum on Friday and Saturday, according to a local government official and witnesses.
One security source told Reuters 25 cars full of rebels had arrived in the capital after the clashes in North Kordofan.
The army said the curfew was to preserve the safety of the civilians and the situation was under control.
"We are announcing a curfew in the state of Khartoum from 5pm (1400 GMT) until 6am starting from today May 10, 2008," an army spokesman said on state television.
Army helicopters flew overheard and the roads shut down in Khartoum as the curfew began to take effect.
Khartoum airport was taken over by Sudan's army. One eyewitness said he saw three Egyptian fighter planes and one Egyptian army cargo plane landing at the airport. The witness said he could see the Egyptian flag on the side of the planes.
The shooting in Omdurman could be heard down the telephone of one resident who telephoned Reuters on Saturday.
"It's all green here because of the military uniforms. There is a lot of army on the streets, security men and military trucks," another witness in the suburb said later.
Diplomatic missions held emergency meetings early on Saturday. They have been on alert since Friday morning. The main phone network crashed in the capital because it was overloaded.
Khartoum houses the bulk of Sudan's population with an estimated 8 million people living in the state. Despite civil wars ravaging Sudan's peripheries for decades, the capital has remained a haven of safety with armed clashes unheard of.