Anti-terror police have been granted three more days to question eight young Muslims arrested on Tuesday. The extension came as a man was charged in Canada with aiding terrorist activity in London.

As the police investigation continued in London, Sheikh Omar Bakri Muhammad, the leader of the radical group al-Muhajiroun, said that Muslims could not co-operate with the authorities against other members of the faith.

The remarks came a day after the Muslim Council of Britain called on the Islamic community to play its part in fighting terrorism. Al-Mujiharoun has been accused of trying to recruit Muslim youth and take over the local mosque in Crawley, West Sussex, where at least three of the suspects were arrested.

The men being held in London, all British citizens of Pakistani origin, have been in custody since Tuesday morning when they were arrested during 24 raids across south east England. The arrests came as police found half a ton of ammonium nitrate fertiliser, which they believe could have been used in a devastating blast.

Last night officers were granted an extension to hold the men under the Terrorism Act until Saturday afternoon, Scotland Yard confirmed. The eight suspects, aged 17 to 32, are held on suspicion of "being concerned in the commission, preparation or instigation of acts of terrorism". Under the Terrorism Act police can apply for extensions to detain suspects up to a maximum of 14 days.

Sheikh Omar, who has denied accusations that his group recruited four young men from Crawley to fight for the Taleban in Afghanistan, told the BBC Radio Today programme: "Muslims have a unique way of life. Co-operating with the authorities against any other Muslims, that is an act of apostasy in Islam.

"Having said that, Muslims in Britain have the right to defend themselves, but without the use of violence. They can gather together, speak with the imams. The imams themselves should lecture the Muslim community, not the Muslim Council of Britain who are all of them a bunch of seculars."

The 44-year-old father of seven also accused Britain and the US of being involved in terrorism.

"I believe the culture of terrorism has become the fashion of the 21st century when the USA and Britain are involved in terrorist activity in Afghanistan and Iraq. Vice versa, al-Qaeda are involved in terrorist activity in Manhattan and Madrid.

"At the end of the day, this is a cycle of violence. You need to look from both sides, you cannot ignore it."

Sheikh Omar, who denounced all violence in the interview, also complained that the media was "demonising" the Muslim community in Britain and that the high-profile arrest of the eight suspects had not helped relations with the Muslim community.

"The British authorities have the right to stop anybody they believe is committing crime. I'm not arguing that. But the way they did it, in very high profile fashion, and giving press details about it, it is clearly sending another message. The British Government is under tremendous pressures from foreign governments to tackle the voice of Islam which remains in the UK."

He also predicted that those arrested would be "released soon".

Shahid Malik, a member of Labour's governing National Executive Committee, told the same programme that the Muslim Council of Britain's actions had been "very appropriate, very responsible".

Mr Malik said: "We cannot live in a fantasy world. Half a ton of ammonium nitrate was found. I think that has been a big wake-up call and a shake-up call, and I think that has been the catalyst for the action of the Muslim Council of Britain."

Computer student Omar Khayam, 22, his brother Shujah Khayam, 17, and cousin Ahmed Khan, 18, from Crawley, Sussex have been named as three of the eight men being questioned at the high security Paddington Green police station in London.

Relatives of Mr Khan and the Khayam brothers have defended the trio, saying that they have done nothing wrong. Mr Khan's father Ansar Khan, 48, a taxi driver who works at Gatwick Airport, claimed that young Muslims were being brainwashed with "wrong" teaching in certain mosques in London but denied that his son and nephews were terrorists.

Anti-terrorist police have said that the alleged London bomb plot was not linked to the March 11 Madrid bombings which killed 191 people.

In Ottawa, Mohammad Momin Khawaja, 29, a software developer who was arrested on Monday is alleged to have participated in or contributed to the activities of a terrorist group and facilitated a terrorist activity between November last year and March 29 in Ottawa and London.

Mr Khawaja, a Canadian of Pakistani descent, recently travelled to London but said that he was meeting a prospective bride. He was remanded in custody until Friday.