Hassan Abdallah al-Turabi is a Sudanese opposition leader and prominent Sunni Muslim, who advocates an Islamic state and is critical of Western secularism.
Hassan al-Turabi is seen as the man who introduced Sharia in Sudan
During an active political life of some 40 years, he has been imprisoned or held under house arrest on several occasions.
Those times in detention have been interspersed with periods of high political office.
His long-running power struggle with his erstwhile ally President Omar al-Bashir shows no signs of ending.
His latest arrest comes after calling on Mr Bashir to hand himself over to the International Criminal Court whose chief prosecutor wants to indict him for war crimes in Darfur.
In recent years, Darfur has been a sore point between the men, and Mr Turabi has openly criticised government policy in Darfur, where pro-government militias have forced more than two million people to flee their homes.
Last year, he was arrested following attacks by a Darfur rebel group, the Justice and Equality Movement (Jem), near the capital.
Jem leader Khalil Ibrahim used to be one of Mr Turabi's followers.
Mr Turabi spent 16 months in prison in 2004-05, during which he spent several weeks on hunger strike.
This followed accusations that he had been involved in a plot to overthrow Mr Bashir, which he denied.
Mr Turabi was one of President Bashir's closest advisers after the 1989 coup which brought him to power.
But the pair fell out following the introduction of a bill to limit the president's powers in 1999, a move which the president resisted by dissolving parliament and declaring a state of emergency.
Mr Turabi was chairman of Mr Bashir's National Congress Party, but was suspended from this post after calling for a boycott of the president's re-election campaign in 2000.
He subsequently formed the Popular National Congress Party (PNC), which boycotted the parliamentary elections later that year.
He was arrested on charges of attempting to overthrow the government after signing a controversial deal with the separatist SPLA rebels fighting for greater autonomy for South Sudan.
The coup accusations and his arrest in March 2004 came only months after his release from custody in October 2003 when he had spent 32 months in detention.
Born in 1932 in Kassala, northern Sudan, to a Sufi Muslim sheikh, Mr Turabi received an Islamic education before studying in Khartoum, London and finally the Sorbonne in Paris, where he gained a PhD.
He joined Sudan's Muslim Brotherhood and rose to prominence during the popular uprising of October 1964 against President Ibrahim Abbud.
President Omar al-Bashir fell out with his former ally in 1999
Between 1964 and 1969, Mr Turabi was the secretary-general of the Islamic Charter Front, a political arm of the Muslim Brotherhood that advocated an Islamic constitution and opposed communism.
Following the 1969 coup that brought General Ja'far al-Numayri to power, he spent six years in custody before escaping to exile in Libya for three years.
In 1979, as President Numayri sought rapprochement with Islamist leaders, Mr Turabi was appointed attorney general.
While he was attorney general, a liberal politician and theologian Mahmoud Mohammed Taha was put on trial for apostasy - abandoning religious belief - and executed in January 1985.
Many Sudanese believed Mr Turabi was behind the president's introduction of certain aspects of Sharia (Islamic Law) and the replacement of the Council of Ministers with a Presidential Council.
Shortly before the 1985 coup that overthrew President Numayri, Mr Turabi was sacked, tried for sedition with other leaders of the Muslim Brotherhood and imprisoned.
After the coup, he dissolved the Islamic Charter Front and reorganised it into the National Islamic Front (NIF).
The NIF's strong finances and support amongst graduates helped the party to third place in the 1986 elections.
Mr Turabi was appointed minister of justice and attorney general in May 1988, and minister of foreign affairs in December 1988.
He also served briefly as deputy prime minister in 1989 before relinquishing all four posts when the NIF refused to endorse a peace agreement drawn up by the government and the SPLA, which advocated secularism in the south.
Although he was imprisoned with other political figures following the 1989 coup that brought to power President Bashir, Mr Turabi was soon released and given a crucial role helping the new government to fashion its policies in accordance with Islam.
During a secret trip to Canada in 1992, Mr Turabi was attacked by an exiled Sudanese martial-arts expert in Ottawa.
He spent four weeks in hospital, and was reported to suffer black-outs resulting from head injuries.
Following the 1996 elections to the new National Assembly, he was elected speaker.
In 1999, Mr Turabi became the secretary-general of the National Congress (NC) Party, which evolved out of the NIF under the leadership of President Bashir.
Mr Turabi is married, and has a son.
His brother-in-law, Sadiq al-Mahdi, is a former Sudanese prime minister, leader of the Ansar religious sect and president of the Ummah Party.