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In the Arab and Muslim world, London has long been perceived as a city friendly to political opposition. It is here that Saudi dissidents set up base in the mid-1990s to escape persecution and irritate the regime with faxes and radio broadcasts.
It is also London where Rached Ghannouchi, the moderate Tunisian leader, chose to make his home, to escape the pressures of a repressive government.
But last week’s attacks have raised questions over whether Britain’s liberal traditions have attracted terrorists and those who encourage them, increasing the threat to London.
Security sources strongly dispute recent suggestions that as many as 10,000-15,000 of the 2m Muslims living in the UK are supporters of al-Qaeda.
Intelligence assessments suggest there are only about 300 British nationals who have gone through or have trained in al-Qaeda camps and are under surveillance, and a core of perhaps 30 unidentified people, either UK nationals or recent arrivals from abroad, among whom could be last Thursday's bombers.
But while many Islamists resident in London have used their base to launch peaceful political opposition campaigns, some have led or joined more militant organisations that could have contributed to the radicalisation of Muslim youth in Britain.
At one end of the spectrum are activists such as Mr Ghannouchi, who often speaks out against terrorism. The government has resisted concerted Tunisian attempts to clamp down on him, insisting that he has not broken British law.
There have also been many Iraqis who worked against the Saddam Hussein regime from their London base. They include Ibrahim al-Jaafari, the current Iraqi prime minister and leader of the Islamist Da’awa party.
At the other end of the scale are people such as Abu Qatada, the Palestinian preacher wanted on incitement of terrorism charges in Jordan. Tapes of his preaching were found in the apartment used by Mohammad Atta, one of the September 11 hijackers. Abu Qatada was arrested in south London in 2002.
Another alleged radical who exploited a base in London is Saudi national Khaled al-Fawwaz. He was accused of having set up a media office in London in 1994 for Osama bin Laden, the al- Qaeda leader. He is now in custody in Britain pending extradition.
Experts on Islamist extremism in the UK say that the more public the activities of organisations based here, the lower the chance of any direct involvement in terrorism.
Public radical groups professing a purely political mission, however, are blamed for stirring enthusiasm for terrorism among impressionable young men.
In some cases, preachers involved in these organisations are radicalised long after they have moved to Britain.
Abu Hamza al-Masri, who used to preach at the Finsbury Park mosque in north London, is a case in point. He was born in Egypt and came to London in the 1970s to study engineering, taking UK citizenship, and then went to Afghanistan to fight the Soviet occupation. His preaching attracted misfits such as Richard Reid, who tried to blow up a transatlantic airliner with a bomb in his shoe.
On the other hand, Syrian-born Sheikh Omar Bakri Mohammed, who moved to the UK in 1985, was the leader of al-Muhajiroun, a radical group that gained a high media profile in the wake of September 11 with inflammatory tactics such as issuing a fatwa, or religious edict, against Pervez Musharraf, the president of Pakistan.
Mr Bakri Mohammed also taught two British Muslims who later mounted a suicide bombing in Tel Aviv.
One former member, Hassan Butt, claimed to have recruited hundreds of young British men to fight for the Taliban from his base in Lahore. The group splintered last year.
The organisation that is gaining a higher profile these days is Hizb ut-Tahrir, a formerly secretive group largely active among students and young people. It is banned in many countries, but legal in Britain.
Imran Waheed, UK spokesman for Hizb ut-Tahrir, denied the group had any involvement in recruiting young people for terrorism, saying it actively discouraged members from pursuing violent means. But he described the London bombings as a “wake-up call” to Britons.
He said: “We have duty to those who lost their lives to ask why the actions of Western governments have caused such hatred.”
The government has come under repeated pressure from Arab countries particularly Egypt, Tunisia, Algeria and Saudi Arabia to expel Islamist dissidents.
But it has sought to balance security concerns against respect for the human rights of dissidents.
Arab regimes have tended to label all their opponents as terrorists, and win convictions against them in courts that lack independence.
In many cases, sending wanted activists home would almost certainly subject them to torture.
In 1995, after a series of bomb attacks blamed on Algerians in Paris, criticism of the allegedly liberal attitude of the British authorities also emerged in France. The Algerian and French press were the first at the time to dub the British capital “Londonistan”.
There are still two extradition cases related to the attacks pending in the UK. But French officials say the level of co-operation between the French and British governments on terrorism has been significantly strengthened.
Even before the September 11 attacks in the US, restrictions on Islamist radicals were being tightened, reducing the attractiveness of the city as a base
for opposition. In February 2001, 21 international organisations were declared illegal in the UK, most of them Islamist.
“Over the years the British perception of this issue has changed,” said a French official.
“And [laxity in Britain] is certainly no longer the feeling of people who are responsible for combating terrorism in France.”