Sudanese authorities have arrested opposition leader Hassan al-Turabi at his home, according to his secretary.
Al-Turabi has denounced last month's elections as fraudulent and said his Popular Congress Party would not join a future government with the ruling National Congress Party.
"At around midnight [2100 GMT], a group of security officers arriving in three cars, came and took Hassan al-Turabi from his home," Awad Babakir, the secretary, told AFP news agency on Sunday.
The arrest comes just days after the Sudanese government renewed calls for the arrestof Khalil Ibrahim, the head of Justice and Equality Movement(Jem), a leading Darfur anti-government group, over a 2008 raid on the city of Omdurman.
Al-Turabi is accused by Sudanese authorities of having ties with Jem and of seeking to ally himself with groups fighting against government forces.
He was also detained for questioning in May 2008 over the Jem attack in Omdurman.
Just hours after Turabi was arrested, the offices of his party's newpaper were raided and its editor-in-chief detained.
"After the arrest of the secretary general of the Popular Congress Party, security authorities stormed the printing house of the daily Rai al-Shaab and confiscated all copies of the paper," Kamal Omar, the PCP politburo chief, said.
"Authorities then took over the newspaper offices in central Khartoum and ordered all the journalists to leave."
Opposition parties threatened on Sunday to take to the streets to protest against al-Turabi's arrest.
"We will do everything we can within the law, sit-ins, demonstrations," Abdallah Hassan Ahmed, Al-Turabi's deputy, said. "We will embarrass this regime and expose it before the international community."
A coalition of 17 opposition parties signed a statement condemning the arrest as a "violation against freedoms and democratic transformation and the constitution".
Siddig Yussuf, the head of the communists, said his party would "stand shoulder to shoulder to remove this regime".
Al-Turabi did not run in April's presidential race. The PCP was instead represented by Abdallah Deng Nial, a Muslim from the mainly Christian south.
|Al-Turabi described last month's elections in Sudan as 'fraudulent [Pete Muller]
"The voting and the counting process are fraudulent," al-Turabi said when votes were being counted.
"We will take the matter to court and if the judge does not rule in our favour, we may have to use other alternatives than the ballot boxes."
Al-Turabi was once an ally of Omar al-Bashir, the Sudanese president, but is now one of his fiercest critics.
Al-Bashir was declared winner in Sudan's first multi-party election since 1986, a vote marred by an opposition boycott, logistical problems and accusations of fraud.
Al-Turabi was last detained in January 2009, two days after he urged al-Bashir to surrender to the International Criminal Court.
In March 2009, the world court issued an arrest warrant against al-Bashir for alleged war crimes and crimes against humanity in the war-torn western region of Darfur.
The seven-year conflict in Sudan there has left 300,000 people dead according to the United Nations, though northern Sudanese authorities puts the figure at 10,000.