Hassan al-Turabi fell out with President Bashir in the 1990s
Sudanese Islamist leader Hassan al-Turabi has been arrested after calling on Sudan's president to hand himself in to face war crimes charges.
The veteran opposition leader is the most high-profile Sudanese figure to say Omar al-Bashir should go to The Hague to face charges over Darfur.
Mr Turabi's son said he was worried for the health of his 76-year-old father.
International Criminal Court (ICC) judges are deciding whether to issue an arrest warrant for Mr Bashir.
The BBC's Amber Henshaw in the capital, Khartoum, says tension is mounting ahead of the ICC decision.
The head of national intelligence recently said foreigners in Sudan could be attacked if an arrest warrant is issued for the president.
Meanwhile, senior government officials have been saying they have intelligence to suggest that a Darfur rebel group, the Justice and Equality Movement (Jem), will launch a series of attacks ahead of an ICC ruling.
A spokesman at Sudan's London embassy said the arrest came as no surprise as Mr Turabi was known to have links with Jem, which he described as a "terrorist organisation".
"This is quite normal as you know in many countries including the United States… people can be arrested for investigation for up to 40 something days," Khalid al-Mubarak told the BBC's Focus on Africa programme.
Mr Mubarak dismissed critics who portrayed the Sudanese government as repressive.
"Our government has signed the Comprehensive Peace Agreement and ended 21 years of civil war; our government in 2006 signed the Darfur Peace Agreement, rebel groups, including the one sponsored by Mr Turabi, have refused to sign and continue to fight.
President Omar al-Bashir could face war crimes charges
"The problem is not our government; the problem is the rebels and the people who sponsor the rebels and embolden the rebels."
Jem leader Khalil Ibrahim used to be one of Mr Turabi's followers.
Mr Turabi was last arrested in 2008, after Jem launched a surprise raid near the capital Khartoum.
Mr Turabi was taken from his Khartoum home just after 2300 local time (2000 GMT) on Wednesday, family members said.
His son, Siddig al-Turabi, told the BBC that he is concerned that intense interrogation could be bad for his father's health.
"Imagine, someone above 70 years detained at this time and questioned for so many hours by maybe more than one committee, which is normal practice. It will tax him heavily," he said.
He said that his father had been hospitalised five times during the three years he spent in prison at the start of the decade.
Mr Turabi's secretary, Awad Babakr, said about 20 security officers arrived at the house in two cars and took him away.
"They didn't give any details, only ordered him to go with them."
A senior official of Mr Turabi's Islamist Popular Congress Party was also reported to have been arrested late on Wednesday.
Mr Turabi said on Monday that President Bashir should hand himself over to the ICC to save the country from possible UN sanctions.
"Politically we think he is culpable.
"He should assume responsibility for whatever is happening in Darfur, displacement, burning all the villages, rapes, I mean systematic rapes, continuously, I mean on a wide scale and the killing."
He added: "Six million of the Sudanese are now paralysed, no agriculture, no animal farming or rearing. He is responsible and we condemn him."
The ICC's chief prosecutor wants Sudan's president to be charged with war crimes and crimes against humanity, accusing him of supporting the Arab Janjaweed militias accused of ethnic cleansing against Darfur's black African population.
Sudan says any charges would be part of a political plot against its leader.
It says issuing an arrest warrant would further destabilise Darfur, where some 300,000 people have died and more than two million forced from their homes during the six-year conflict.
Mr Turabi used to be a close ally of President Bashir but the pair fell out in the 1990s.