All operations and flights at Khartoum airport have been suspended after unrest erupted in the Sudanese capital, the UAE-based Sky News Arabia TV reported.
Security forces attacked a pro-democracy protest outside the military headquarters in the capital, Khartoum, activists say.
Medical sources say at least eight people have been killed, with reports of live ammunition being used.
Sudan has been governed by a Transitional Military Council (TMC) since President Omar al-Bashir was overthrown in a coup in April.
The council later denied using force to break up the main protest site.
“Sudanese forces did not disperse the sit-in outside the army headquarters by force, but rather targeted a nearby area which has become a threat to the safety of citizens,” TMC spokesman Lt Gen Shams al-Din Kabbashi told UAE-based Sky News Arabia TV channel.
Protesters have been demanding that a civilian government take over the running of the country.
The security services moved on the main protest site early on Monday, activists say.
Khartoum residents reported hearing heavy gunfire and seeing smoke rising from the area around the army headquarters.
A source inside one of the main hospitals in Khartoum told the BBC they had received at least eight bodies and many others injured.
The Central Committee of Sudanese Doctors put the death toll at nine.
“Now an attempt is taking place to disperse the sit-in,” said a short statement from the Sudanese Professionals Association, the group which is spearheading nationwide protests.
The association also called for a campaign of “sweeping civil disobedience to topple the treacherous and killer military council and finalise our revolution”.
Protesters have been burning tyres and erecting barricades to try to fend off the security forces.
Journalist Benjamin Strick, who specialises in verifying footage on Twitter, has shared dramatic videos from Khartoum, where repeated gunfire could be heard.
The TMC denied using force against the sit-in.
Lt Gen Shams al-Din Kabbashi said the security forces had moved towards Colombia, a neighbourhood located near the main protest site, which he said “has long been a hotbed of corruption and negative practices that contradict the conduct of the Sudanese people”.
“The area has become a major security threat to our citizens… and it also affects the safety of protesters in the sit-in area.”
“We did not disperse the sit-in by force. The tents are still there and the youth are moving there freely,” the TMC spokesman added, stating that many protesters “preferred to leave the sit-in”.
Meanwhile, the US embassy in Sudan said attacks by Sudanese security forces against protesters were “wrong and must stop”.