The UK terror threat level has been raised to its highest level of "critical", meaning further attacks may be imminent, Theresa May has said.
The move came after investigators were unable to rule out whether Manchester bombing suspect Salman Abedi acted alone, the prime minister said.
Military personnel will now be deployed to protect key sites.
Twenty-two people were killed and 59 injured when a suicide bomber attacked concert-goers at Manchester Arena.
The prime minister also confirmed the government had triggered "Operation Temperer", a long-standing emergency plan to put soldiers into key public locations to support armed police in protecting the public.
Military personnel may also be seen at other events over the coming weeks, such as concerts, Mrs May said, and would work under the command of police officers.
The highest level has only been reached twice before.
The first occasion was in 2006 during a major operation to stop a plot to blow up transatlantic airliners with liquid bombs.
The following year, security chiefs raised it once more as they hunted for the men who had tried to bomb a London nightclub, before going on to attack Glasgow Airport.