Counter-terrorism detectives have been given more time to question four men arrested on suspicion of spying for Iran at locations and individuals linked to the Jewish community.
The suspects, one Iranian and three dual British-Iranian nationals, can now be held in custody until March 13, the Metropolitan Police said on Saturday.
The men, aged 22, 40, 52 and 55, were arrested under the National Security Act at addresses in Harrow, Watford and Barnet shortly after 1am on Friday.
The Met said six other men, aged between 20 and 49, arrested at the same location in Harrow, have been released on bail pending further investigation.
One of the men was also arrested on suspicion of assaulting a police officer, the force added.
Detectives are understood to be investigating why the alleged surveillance of Jewish locations and individuals was being carried out and whether it was linked to a desire to carry out attacks on British soil.
Part of the investigation looks into claims that in-person surveillance took place in London and whether it was directed from abroad. The anti-terrorist police and MI5 operation had been underway for months.
Shabana Mahmood, home secretary, said on Friday the arrests meant police had kept “Britain safe from a potential threat”.
Mahmood said: “I want to thank the police and our security services for the measures they have taken today to keep Britain safe from a potential threat.
“It is understandable that the Jewish community and the general public are concerned about today’s arrests. We continue to closely monitor the situation and engage with those affected.”
He added: “I can assure you that our police and security services are world-leading and will not hesitate to take action to counter any threat to the UK. They will continue to use the full range of tools and powers available to them to keep this country safe. They have the full support of the Government as they carry out their vital work.
“We must now give them the time and space to continue their investigations.”
In October last year, MI5 director-general Ken McCallum said 20 Iran-linked plots had been thwarted on British soil in the previous 12 months. Most of them were against Iranian dissident targets, and at least one is linked to an Israeli target in Britain.
