Four are Iranian citizens and the nationality of the fifth was still being established.
Police said the attack plot targeted a single location that was not being named “for operational reasons.” It said the premises was being given "advice and support."
All the suspects were being questioned at police stations and have not been charged. Police said they are searching several properties in London, the Manchester area of northwest England and Swindon in western England.
Forensic officers in blue overalls were photographed at a house in Rochdale, Greater Manchester, where one of the men was detained. Three of the counterterror arrests took place in the Greater Manchester area, one in London and one in Swindon.
Rochdale resident Kyle Warren told Sky News he “heard a massive bang” and saw “20 or 30 police with guns” drag a man from a neighboring house.
“We’ve seen a man getting pulled out from the back, basically got dragged down the side entry and thrown into all the bushes and then handcuffed,” he said.
Commander Dominic Murphy, head of the force’s Counter Terrorism Command, said police are still working to establish a motive “as well as to identify whether there may be any further risk to the public.”
Separately, three other Iranian men, aged 39, 44 and 55, were arrested in London on suspicion of a national security offense as part of an unrelated investigation, police said.
Home Secretary Yvette Cooper said the arrests were “serious events that demonstrate the ongoing requirement to adapt our response to national security threats.
“The government continues to work with police and intelligence agencies to support all the action and security assessments that are needed to keep the country safe,” she said.
Police have not determined whether the alleged plot is connected to Iran, but Britain's domestic intelligence service has warned of a growing threat from attackers linked to Tehran.
In October, the head of the MI5 domestic security service, Ken McCallum, said his agents and police have tackled 20 "potentially lethal" plots backed by Iran since 2022, most aimed at Iranians in the U.K. who oppose the country's authorities.
He said at the time there was the risk “of an increase in, or broadening of, Iranian state aggression in the U.K.” if conflicts in the Middle East deepened.
In March 2024, Pouria Zeraati, a presenter at a Farsi-language television station critical of the Iranian government, was stabbed in the leg outside his home in London. Two men were later arrested in Romania and charged over the attack.
The U.K.’s official terror threat level stands at “substantial,” the middle of a five-point scale, meaning an attack is likely.
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A previous version of this story said the alleged target was in London. Police have not specified its location.