By Andrew Carey, CNN

London (CNN) - Usama Hasan has led Friday prayers at Leyton mosque in East London for 20 years.

Back in the early 1990s, his sympathies lay with what became known as the global jihad and he was an active supporter of extremist causes.

Over time, however, he warmed to Britain and started questioning the idea that the West was always acting against Muslim interests.

Now, his message is different. In recent years he has used his position as an imam to tackle issues like terrorism head-on.

But it’s his views on Darwinian evolution that have landed him in trouble with some fellow Muslims. It’s been two months now since he says criticism over his support for the theory of evolution provoked him to stop leading Friday prayers at his congregation.

Besides being a religious scholar, Hasan is a scientist who has studied theoretical physics at Cambridge and who is a senior lecturer in engineering at Middlesex University.

Two and a half years ago he wrote an online article decrying what he called the "appalling state of science in the Muslim consciousness."

He described the belief that Adam was made from clay, as the Quran suggests, and then made a living human after God breathed life into him, as a "children's madrassa-level understanding" of the origins of man.

The remarks prompted some in the community to turn against him.

“People weren't reasoning with me,” he says, “rather, they would say, ‘you're no longer a Muslim, you're an apostate, you're an infidel.’”

According to Hasan, his opponents started seeking fatwas, or religious rulings, from clerics overseas to denounce his support for evolution and, he says, to declare a death sentence.

Hasan says that clerics in Pakistan, Kuwait and Saudi Arabia issued such decrees.

Leaflets started appearing at the mosque calling for Hasan’s removal and making thinly veiled suggestions that his "apostasy" might ultimately sanction his execution.

A public meeting aimed at calming the situation only increased tensions. Recently, Hasan says he started fearing for his life.

“Given the fact we have many young Muslims in this country who have taken law into their own hands, with terrorist plots, with a plot to murder an MP last year, I do feel that somebody young and impressionable may feel it their religious duty to kill me,” he told CNN.

Last November, London student Roshanara Choudry was found guilty of trying to kill Stephen Timms, a member of parliament, by stabbing him with a knife. It emerged during her trial that she had been influenced by extremist Muslim clerics online, including the U.S.-born preacher Anwar al-Awlaki.

Police confirmed to CNN they are investigating alleged threats against Hasan.

But recent weeks have seen campaigns launched by fellow Muslims to support Hasan.

Tehmina Kazi of the group British Muslims for Secular Democracy started a Facebook group that garnered eight hundred members in its first 24 hours.

And a senior group of Muslim clerics and activists signed a letter to the Guardian newspaper decrying Hasan’s treatment.

One of the signatories, Sheikh Ibrahim Mogra, told CNN he was very worried at how the practice of "takfir" - declaring individual Muslims to be apostates - had taken hold in some elements of the British Muslim community.

“There have been volumes and volumes written to regulate this kind of behavior,” Mogra said. “It's not for any Muslim to go about and declare (an apostate) ... judgment is for God.”

Earlier this month, Hasan issued a statement on his website dialing back some of his statements on evolution.

"I regret and retract some of my statements in the past about the theory of evolution, especially the inflammatory ones … I do not believe that Adam, peace be upon him, had parents," he wrote.

"Some of the things I said in public went too far, and without meaning to, had been quite inflammatory,” he told CNN.

“What I would like to point out now is that religious scholars, in the main, are opposed to evolution, they believe it is blasphemy, it is against the Quran," he said. "Whereas scientists say that evolution is a scientific fact, or a scientific theory with overwhelming evidence."

"There exists this gulf, this impasse," Hasan continued. "And at some point people may have to address that issue, but only when the community and the religious scholars are ready.”

Mogra, who does not share Hasan’s previous position on evolution but supports his right to discuss it, is worried about the debate.

“This is a critical issue for us - if we fall at this hurdle then who's next tomorrow?" Mogra said. "If we are going to allow ourselves to be intimidated like this and publicly retract and say things we don't actually believe in, then that would be a great disaster for Muslim scholarship and for the freedom that we enjoy in this country.”

Hasan says that he hopes the current tensions dissipate and that he can return to what he calls his favorite mosque in London to join in prayers.