Hassan al-Turabi, a prominent Sudanese politician considered by many to be the spiritual leader of the country's Islamists, has died at the age of 84, medical sources said on Saturday.

A powerful establishment figure in the 1990s, Turabi became an influential opposition leader later in life, breaking ranks with President Omar al-Bashir and his ruling National Congress Party in 1999 to form an opposition group that would challenge his rule.

Turabi was rushed to hospital on Saturday after suffering a heart attack, the sources said.

In recent years he played a role in Bashir's attempts to bring sparring parties together through a national dialogue process called for in 2014 but which has seen little progress.

Turabi's Popular Congress Party (PCP) was considered the most prominent opposition group participating while many others have boycotted the process.

"Turabi's sudden absence from the political field will have a disruptive effect on the national dialogue that President Bashir has called for...With his death, the national dialogue has lost its biggest supporter," said Mohamed Latif, a Sudanese political analyst.

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Turabi published numerous books on Islamist thought and spearheaded the National Islamic Front, a group he formed in the 1970s to push for the Islamisation of the state.

"Turabi's death will have serious repercussions for Islamists, as he is the thinker and godfather of the Sudanese Islamist movement," said Latif.

Turabi was a close ally of President Bashir throughout the 1990s, serving as his foreign minister just after a 1989 military coup that brought Bashir to power. He was elected speaker of parliament in 1996.

In 1999, following differences between him and Bashir, Turabi formed the PCP, taking on an opposition role that landed him in jail several times.

Opposition groups accuse Bashir of jailing dissidents and censoring the media.

Bashir has remained in power for more than a quarter of a century, weathering rebellions, economic crisis and an indictment by the International Criminal Court on suspicion of having orchestrated war crimes in Sudan's Darfur region.

Turabi, the son of a local imam, was born in 1932 in Kassala in eastern Sudan and moved to Khartoum to study law before completing his studies in London and Paris.

He joined Sudan's Muslim Brotherhood and rose to national prominence in the 1964 revolution which overthrew General Ibrahim Abboud.