Sudanese authorities are reported to have shut down the headquarters of the party of the main opposition Islamist leader Hassan al-Turabi.
Turabi was released from detention last October
Mr Turabi was detained along with several opposition politicians and army officers over an alleged plot to overthrow President Omar al-Bashir.
He has denied his party - the Popular Congress Party (PCP) - was involved.
The government said the group had been "planning acts of subversion" at a number of strategic establishments.
Defence Minister Bakri Hassan Salih alleged that the officers had been planning the coup since the middle of last year, assisted by the PCP.
Officials have also accused the alleged coup-plotters of links with rebels in the western province of Darfur.
On Wednesday talks between the government and the Darfur rebels got under way.
Although the government delegation and a number of rebel representatives failed to appear at the opening ceremony in the Chadian capital, Ndjamena, on Tuesday, the opposing sides are now involved in indirect talks.
Mr Turabi told the BBC's Focus on Africa programme before his arrest said there had been "tension within the army" but there had not been an attempt to overthrow the government.
He said he felt the government was trying to aggravate the situation to prepare the ground for his arrest and to ban the PCP completely.
The opposition leader is a former ally of Mr Bashir, who came to power in an Islamist-backed coup in 1989.
Mr Turabi was previously detained in 2001 after a power struggle with Mr Bashir - and was released from house arrest in October last year.
The BBC's Alfred Taban in Khartoum says the city's former governor, Badr-Eldin Taha has gone into hiding, accused of supporting the Darfur rebels.
Hundreds of thousands of civilians have been displaced, with more than 100,000 fleeing across the border into Chad.
Thousands have been displaced by the fighting in Darfur
The Sudan Liberation Movement (SLM) and Justice and Equality Movement (Jem) rebels say the government has been oppressing blacks in favour of Arabs.
The United Nations humanitarian co-ordinator for Sudan, Mukesh Kapila, said that government-backed Arab militias have been systematically raping and killing in Darfur.
The fighting in the west of Sudan has intensified as government peace talks to resolve the 20-year war with southern rebels near an end.
The UN is concerned this conflict could undermine the talks which are taking place in Kenya.