LONDON — Four men were arrested in Britain on Friday on suspicion of spying for Iran on locations and individuals linked to the Jewish community in the London area.

One Iranian and three dual British-Iranian nationals were arrested shortly after 1 a.m. in areas north of the capital as part of a preplanned operation, counterterrorism police said.

The men were detained on suspicion of assisting a foreign intelligence service. “The country to which the investigation relates is Iran,” the police said in a statement.

“Today’s arrests are part of a long-running investigation and part of our ongoing work to disrupt malign activity where we suspect it,” Cmdr. Helen Flanagan, the head of counterterrorism policing in London, said in the statement.

“We understand the public may be concerned, in particular the Jewish community, and as always, I would ask them to remain vigilant and if they see or hear anything that concerns them, then to contact us.”

The four men arrested included a 40-year-old and a 55-year-old arrested at addresses in the Barnet area, police said. Searches continued at those locations and another address in the area. The other two were a 52-year-old arrested in Watford, northwest of London, and a 22-year-old arrested in Harrow. Both locations were being searched, as was a location in the Wembley district.

Six other men who were not targeted in the operation were detained in the Harrow area on suspicion of trying to assist one of the men who was being arrested.

British Prime Minister Keir Starmer offered his thanks to “the police, the security services, and everyone involved in keeping Britain safe.” In a statement on social media, he added: “People will be worried after today’s arrests, especially those in our Jewish community. Our world-leading security services will not hesitate to protect you.”

Despite the outbreak of war in the Middle East, there has been no change to Britain’s national terror threat level, which stands at “substantial” — meaning an attack is probable. (There are two higher levels — “severe” and “critical.”)

On Sunday, the British defense secretary, John Healey, highlighted the increased risk of indiscriminate retaliatory attacks by Iran and its proxies, adding that the domestic threat assessment was under review and that vigilance in Britain was high.

Britain’s intelligence services have previously named Iran among the three countries posing the greatest threat to the country, alongside Russia and China.

Iranian reporters and broadcasters based in Britain have suffered physical attacks, threats and surveillance. In October, Ken McCallum, the director general of MI5, the domestic security service, said his organization had “tracked more than 20 potentially lethal Iran-backed plots in just the one year.”