The move followed Wednesday's Golders Green stabbings of two Jewish men in north London and warnings of broader Islamist, extreme right-wing and state-linked threats.
Police said suspect Essa Suleiman, 45, remains in custody, had been referred to Prevent in 2020, and may be linked to a separate south-east London incident.
The severe level, last used in 2022, prompted extra armed patrols in London and follows a series of recent attacks on Jewish people, synagogues and community property.
Iran-linked groups now claim UK attacks. Is Britain facing a new era of state-sponsored terror on its own soil?
The London attacker was a known violent extremist. How many more have slipped through the cracks of the UK's security net?