Sudanese security officials say they have foiled an opposition plot aimed at creating “security disturbances” in the capital, Khartoum.
Witnesses described seeing tanks and troops in the centre of the city in the early hours of Thursday.
Sudan has seen demonstrations in recent months over economic hardship, triggered by a dispute over oil revenues with South Sudan.
A deal forged in September on ending the dispute has been delayed.
Armoured vehicles and tanks were seen in the early hours of Thursday in a main street in central Khartoum, although news agencies said there was no increase in security later on.
“This plot is led by some opposition party leaders,” the Sudanese Media Centre reported on its Arabic-language website.
Military and civilian figures had been arrested in connection with a plot to destabilise the country, the report said.
Although Sudan has witnessed protests in recent months calling for the end of President Omar al-Bashir’s 23-year regime, they have since died out after a number of arrests of opposition figures.
In September, Sudan agreed with South Sudan to halt fighting and resume oil exports from the South via Sudan.
However, the two neighbours have failed to implement the terms of the accord and South Sudan has accused Sudan of carrying out air strikes over the border, an accusation denied by Khartoum.
The Sudanese army said it was targeting rebels from the Darfur region well inside its territory and accused the South of supporting the rebels.
Earlier this month, another rebel group – the Sudan People’s Liberation Movement-North – said it had shot down a government military aircraft on the disputed border between Sudan and South Sudan.