A number of mosques across the UK have appealed to worshippers at Friday prayers to co-operate with the police in the fight against terrorism.
Friday prayers included a message of co-operation
The Muslim Council of Britain (MCB) urged imams to make the appeal but some are thought to be angry their religion has been linked with terrorism.
Meanwhile, police are questioning a ninth man following anti-terror raids in the South East.
Tony Blair welcomed the appeal and said new anti-terror laws could be drawn up.
At Regent's Park Mosque, the largest in the country, spokesman Abdesselam Daoud said although the MCB's letter would not be read out in full, its sentiments and concerns would be reflected in the day's sermon.
"It's not practical to read a letter to a large audience but the sermon will focus on concerns of brotherhood and security of the community," he said.
But as several thousand prayer-goers left the London mosque, fringe radical group Al Muhajiroun staged a protest and burning of the union flag.
The overwhelming majority ignored the protest, while some of the prayer-goers shouted abuse at the radicals.
Others complained that the media at the scene were hyping the situation.
During his Downing Street briefing to press on Thursday, Mr Blair indicated identity cards would be brought in soon and further anti-terrorist legislation was being drawn up.
The MCB drew up a letter to mosques in the wake of the Madrid bombings, although news of its unprecedented step came at the same time as the raids in the South East, which led to the seizure of half a ton of fertiliser used in bombings.
Police have until Saturday to question the first eight men - thought to be Britons of Pakistani descent - arrested over an alleged bomb plot.
The latest man to be arrested was a 27-year-old Briton held in Crawley, West Sussex - the fourth in the town.
He was held on Thursday evening on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism.
A man has also been detained by police in Canada in connection with alleged terrorist offences in London, although police have not indicated whether the arrest is linked to the British raids.
Mohammed Momin Khawaja is due to appear via video link before a court in Ottawa on Friday.
The Muslim Council of Britain said there had been an "overwhelmingly positive" reaction to its calls for mosques to help in the fight against terror.
And it dismissed the comments of Sheikh Omar Bakri Muhammad, a cleric who on Thursday said the MCB was being unfair and advised Muslims not to co-operate.
MCB general secretary Iqbal Sacranie said: "The message that is going out is not in any way associating mosques with terrorism."
Speaking on the BBC Radio 4 Today programme, he said: "Mosques are an
important institution in the country.
"The imams and chairmen and secretaries are playing a very important role in
"We are facing a major crisis in the country and world over. We have to
exercise our duty, an Islamic duty, which is to convey the message to the
community that they have responsibilities as well."
As well as Friday's sermons, booklets are being printed that will remind Muslims of their obligation to help safeguard Britain's security.