"All four have been arrested on suspicion of the commission, preparation and instigation of terrorist acts," the police said, giving no details on the target of the apparently thwarted attack.
Armed police officers patrol outside Westminster underground station the morning after an attack in London, Britain, March 23, 2017.
London Police Release All 12 People Arrested Over March 22 Terror Attack
The woman in her twenties, who was shot, was also a subject of the investigation. She was taken to hospital and is under police guard, pending recovery.
The London police stressed the incident was not connected to the recent arrest of a knife-wielding man near the UK houses of parliament, which happened a month after another man stabbed a police officer to death outside Westminster.
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