The official Sudan Media Centre said on Saturday the government had lodged a complaint against al-Turabi, detained on Wednesday with members of his Popular Congress party and some military officers, for an alleged plot to overthrow President Umar al-Bashir.
It is the first time the government has accused the detained Islamist leader of involvement in specific criminal acts. Al-Turabi is expected to be questioned further regarding the accusations before authorities decide whether he should face trial.
The head of al-Turabi's legal team, Ali Mahmud Hasanain, criticised his arrest and called for a "fair, just and open trial" if the case goes to court. The media centre said a Khartoum criminal law attorney would handle the case against al-Turabi.
Human rights activist Ghazi Sulayman denounced the arrests and criticised the government for denying family members and lawyers access to the detainees.
"As to the saying that [Turabi] is behind the coup, I have yet to see any sign of a coup in the streets of Khartoum and I have yet to see the government providing us with names and proof," said Sulayman, head of the independent Sudanese Human Rights Group.
Al-Turabi's party had denied any involvement in the alleged coup attempt. The party said the alleged plot involved members of the security forces in the western province of Darfur, where local tribes have been in revolt since early 2003.
Al-Turabi was the main ideologue of the Islamist government that was set up after al-Bashir seized power in 1989.
He became one of the African country's most powerful figures during the next decade, when Sudan was a haven for foreign Islamist groups, including al-Qaida leader Usama bin Ladin.
Friend turned foe: Umar al-Bashir
was helped to power by al-Turabi
Al-Turabi said in 1998 the US embassy bombings in Kenya and Tanzania were "understandable" and he considered Usama bin Ladin a hero.
But intense international pressure on Sudan to end its alleged ties with terrorism and the country's own domestic problems - led by a two decade-long civil war that has claimed more than two million people - ate away at al-Turabi's influence.
US sanctions ban American companies from doing business in Sudan.
Fall from power
Al-Bashir and al-Turabi fell out in 1999 after the president accused al-Turabi, then speaker of parliament, of trying to grab power and stripped him of his position.
Al-Bashir began to move away from Islamist government - in part, experts say, to get foreign aid and technology to exploit his country's oil resources.
Al-Turabi was freed five months
ago from house arrest
Al-Turabi, meanwhile, formed the Popular Congress and became the most prominent Islamist in opposition.
He spent two years under house arrest after his party signed a peace deal with the rebel Sudan People's Liberation Army, leader of the southern rebellion.
His return to custody on Wednesday comes just five months after his release.
In a recent interview before his arrest, al-Turabi said he expected to be detained because the government wanted to blame him for the conflict in the Darfur region.
UN officials and human rights groups have accused the government of supporting militias responsible for abuses in the troubled region, which Khartoum denies.