Mr Turabi's son Siddig said his father had been freed and returned to his home in Khartoum without explanation.

Mr Turabi was imprisoned two months ago after calling on Sudanese President Omar al-Bashir to surrender to the International Criminal Court.

The ICC issued an arrest warrant for Mr Bashir on charges of war crimes and crimes against humanity in Darfur.

"We don't know what is going to happen in the morning but there is no guard outside the house in Khartoum," said Siddig al-Turabi, Reuters news agency reported.

"He has been released... We are very happy."

As leader of the National Islamic Front and speaker of the Sudanese parliament, Hassan al-Turabi was a key ally of President Bashir until they split in a power struggle 10 years ago.

He is the leader of the Islamist Popular Congress Party and has been frequently arrested in the past.

Darfur rebels

He has been linked to the Darfur rebel Justice and Equality Movement, an allegation he has denied.

Last week, the ICC arrest warrant for President Bashir was the first the court has served against a serving head of state.

Mr Bashir has rejected the charges and accused the ICC of colonialism.

The UN estimates that 300,000 people have died and 2.7 million displaced in Darfur, since black African rebels took up arms in 2003 against the Arab-dominated regime demanding a greater share of resources and power.