Sudan's powerful intelligence service on Thursday said it had disrupted a "plot" by members of the military and the political opposition to disturb the country's security.
At about the same time, a witness saw tanks moving in the streets of the capital.
But a spokesman for an alliance of opposition parties denied any link with attempts to bring about violent political change in the country which has already experienced seven coups or attempted coups in its 56-year history.
"The security and intelligence service early today stopped a plot to disturb security," said the Sudanese Media Centre, which is close to the security apparatus.
The Media Centre quoted a source as saying authorities had been investigating both civilian and military personnel.
"This plot is led by some opposition party leaders," the Media Centre said.
"We heard about that but I think it is fake," Farouk Abu Issa, spokesman for the opposition parties, told AFP.
"We are for democratic, peaceful change" through strikes and demonstrations against the 23-year Islamist regime of President Omar al-Bashir, he said.
"The government knows that."
A witness told AFP that he saw troops moving early Thursday, about the same time as the plot was reportedly broken up.
"About 2:00 am (2300 GMT) while passing Obeid Khatim Street I saw some tanks and vehicles with military equipment and soldiers coming from a southerly direction and heading downtown," said the witness, who asked for anonymity.
Obeid Khatim Street is a wide thoroughfare running alongside Khartoum's military and civilian airports, leading into the downtown area where government buildings are located.
However, there were no signs of extra troops downtown later Thursday.
Islamist reformers charge that corruption and other problems have left the government Islamic only in name and question how much longer Bashir should remain in power.
"A lot of people are saying 23 years is too long a time, and what's the difference between him and Mubarak and Assad?" a Sudan analyst who asked for anonymity said earlier.
He was referring to ousted Egyptian president Hosni Mubarak and Syria's President Bashar al-Assad.
In June and July scattered youth-driven protests, initially sparked by high inflation, called for an end to the regime in line with calls by Arab Spring demonstrators throughout the region.
Sudan's protests petered out in the face of widespread arrests.
"They themselves are afraid of their people uprising and saying 'no' to their policies," Issa said, noting that the government has just raised the price of sugar, a Sudanese staple.
Inflation exceeds 40 percent.
"At the same time, there are differences between different factions" in the ruling apparatus, Issa said.
Analyst say the security forces themselves do not uniformly support the regime, which is fighting rebellions in South Kordofan and Blue Nile states, as well as unrest in Darfur, while tensions have resurfaced on the unmarked border with South Sudan.
The report of the "plot" comes as concern grows over delays in implementing security and oil deals which the leaders of Sudan and South Sudan hailed in September as ending conflict, after they fought along their undemarcated border in March and April.
Sudan's army on Wednesday confirmed it attacked an area near the South Sudanese border where Darfur rebels had set up a compound, but South Sudan said bombs landed on its territory, killing five people.
"We attacked Al-Regaibat which is 40 kilometres (25 miles) north of the international border with South Sudan and 10 kilometres north of Samaha," army spokesman Sawarmi Khaled Saad said in a written statement.
Saad alleged that the rebels must have had "great support" from neighbouring South Sudan, an accusation which goes to the heart of tensions between the two nations.