British law enforcement agencies say they have averted a plot to orchestrate a large-scale terror attack similar to the assault on Kenya's Westgate mall.
Police are questioning four men in their 20s on suspicion of terrorism after they were detained on Sunday in pre-planned, intelligence-led raids.
A British security official says the men were planning a shooting spree akin to the Westgate attack in Nairobi, in which at least 67 people died.
The official - who spoke on condition of anonymity because he was not authorised to speak to the media on the record - says it's not clear how advanced the planning was but that any attack was not imminent.
Metropolitan Police did not identify the suspects or say what, if any, charges, they may face. But in a series of statements, the force said the men were all British nationals between the ages of 25 and 29, with roots in Turkey, Pakistan, Algeria and Azerbaijan.
The force confirmed that the London police firearms unit took part in the arrests. British police rarely carry weapons and their involvement suggested concern that men might have been armed.
Dramatic CCTV footage has emerged of one of the arrests, in which counter-terror police are seen tackling a suspect on the street in an upscale west London neighbourhood.
Elsewhere, police fired special ammunition - aimed at breaching doors or bursting tyres - to stop a car near the Tower of London, and arrested two 25-year-old men. Another suspect was arrested in southeast London.Officers were searching six locations and two vehicles in London following the arrests.