The mosque, closed in 2003 because of concern that those in control were too extreme, was reopened last August with new trustees.
But the Guardian has learned that in December hardliners retook control. They are alleged to have enforced their will by violence and intimidation and to have preached sermons which some at the mosque regarded as extreme.
On Saturday trustees appointed last week changed the locks and took physical control of the building.
Police officers went to the mosque in case of trouble, but did not enter it.
Imam Abu Hamza is alleged to have used the mosque to spread extreme views until the Charity Commission demanded his removal, and the mosque was closed for 18 months.
Mr Hamza is on remand in Belmarsh top security prison awaiting trial on a string of charges, including incitement to murder, which he denies.
There is no evidence that he has been involved in recent events at the mosque.
For weeks now Muslim groups have been trying to install a new management in the mosque, technically administered by a charitable trust. It was argued that it was in danger of falling into irrevocable disrepute.
Sources say the Charity Commission became concerned that alleged hardliners were back in control. The commission was involved with the Muslim Association of Britain in selecting and approving the new trustees.
Kamal Helbawy, a spokesman for the new trustees, said: “People were afraid to go in and pray or to do activities. People were afraid of those who had seized the power.”
Dr Helbawy said those who had seized control would be allowed to pray at the mosque but would be banned from giving sermons.
“They are not allowed to preach and teach their own ideas. We consider them extremists, they are advocating violence, they are not preaching the middle way of Islam.”
A senior member of the mosque told the Guardian that he had been surrounded and punched unconscious inside the mosque about a month ago. “I went inside, they just ran at me, I was circled and punched. I woke up after a few seconds, I just walked out. This is not Islam, you can’t tell them anything, you will be bullied and attacked.”
He said another mosque elder had been followed home and attacked. At least two assaults on people opposed to the alleged hardliners have been reported to the police.
A senior police source said: “We had become aware that local people felt intimidated and have not been able to go in. It was all unsavoury and unsatisfactory.”
Massoud Shadjareh of the Islamic Human Rights Commission said: “After the hardliners took over the mosque, its future looked bleak, that’s why this has been done.
“We do recognise there has been problems at that mosque over many many years. The community needs to consulted, be empowered and brought back in control.”
Locals point to dwindling attendance at Friday prayers as a sign of the dire state into which the mosque had fallen: in recent weeks it has been below 100, compared with a peak of 1,500. Another mosque only metres away drew 2,000 worshippers.
A Scotland Yard spokesman said: “Action was taken on Saturday afternoon by the trustees of the North London Central mosque. We are in liaison with the legal owners of the property, the Charity Commission and community representatives.
“An appropriate police presence has been monitoring and will continue to monitor the area to ensure the safety of people using the mosque and prevent any disorder or breach of the peace.”
No one from the alleged hardline group could be reached for comment. They have previously claimed to have been victims of anti-Islamic sentiment.