Sudanese authorities arrested the country's leading Islamist ideologue as they sought to respond to the weekend attack on the capital by Darfuri rebels.

Hassan al-Turabi, the man who invited Osama bin Laden to live in Khartoum during the 1990s, was taken into custody but then released without charge after 15 hours of questioning.

Mr al-Turabi is closely linked to the leader of the Justice and Equality Movement (JEM), Khalil Ibrahim, who was once his pupil.

On Saturday the rebels launched an audacious attack on Khartoum's twin city of Omdurman, the seat of parliament and separated from the capital only by the Nile. They were beaten back before they could cross into Khartoum but today promised fresh attacks.

Security services have responded with a round-up of al-Turabi's supporters. Issam al-Turabi, his son, told The Times that armed soldiers and pickups mounted with anti-aircraft guns surrounded the family home at about 4am.

Mr al-Turabi, who was entrusted with Bin Laden's horse when the al-Qaeda leader was asked to leave Khartoum in 1996, confirmed his father had been released without charge.

He said: "They asked him about the rebellion, whether he knew they were heading to Khartoum and whether he stood side-by-side with them. He refused to answer so in the end they told him they were taking him home."

His father was once the ideological power behind the Government of President Omar al-Bashir. He wanted to turn Khartoum into a haven for Islamist movements from around the world.

Osama bin Laden arrived in the city in 1991 after falling out with the Saudi government, but Hassan al-Turabi was sidelined as President al-Bashir tried to improve relations with the West. Since then he has been imprisoned and subjected to house arrest. In recent years he has been left alone as his influence appeared to wane.

In an interview with The Times before his arrest he said that he supported the rebels' cause and had long advocated a federal structure for Sudan. “These are the people who have had a key role in our economy and the Army,” he said. “The people have contributed to Sudan and they deserve a degree of equality.

He said he had split with his former pupil — the doctor, as he calls him — over the use of force to achieve greater Darfuri autonomy. "The doctor thought there was no point in forming a party as it would be banned, so he took up arms instead,” he said. “But quite a number of our leadership went with his group.”

Many of the JEM leaders were members of al-Turabi's Popular Congress Party and share his Islamist outlook.

Last year the US imposed sanctions against Dr Ibrahim for his role in the violence. He claimed to still be in Omdurman today and promised his assault was far from over.

“This is just the start of a process and the end is the termination of this regime,” he told Reuters by telephone. “Don't expect just one more attack. This is just the beginning.”

The weekend attack, when a Jem convoy managed to drive from Darfur into Omdurman, is the first time the conflict has reached the capital, Kjartoum. Sudan immediately severed ties with Chad, which it said was behind the attack.

The two countries have been fighting a proxy war using opposing sets of rebels. Now the fear is that it could become a real war.

Analysts said that Jem had achieved its aim of undermining Khartoum's reputation as an impregnable fortress, exposing opposition and fear within the capital.

John Prendergast, of the Enough Project which campaigns against genocide, said: “There were more soldiers on the inside than there were soldiers who ventured all the way from Darfur, which should be very worrisome for the Government.”

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