Police in the West Midlands were ordered off the streets after a threat was made to kidnap and kill an officer in Birmingham, Sky News has learned.
Officers were called back to base by West Midlands Police between 10pm and midnight on Monday after an anonymous tip-off.
The force, which has declined to comment on the specifics of the threat, said officers and staff had been given an "additional safety reminder".
Officers were spoken to about travelling to and from work in partial uniform, as well as security at police stations.
The threat level to police across Britain was raised to substantial in October, meaning an attack is a strong possibility.
Assistant Chief Constable Garry Forsyth said: "There is nothing at this time to suggest there is an increased risk to members of the public.
"We have taken the opportunity to remind all employees of the need to be vigilant.
"Our priority remains serving our communities and protecting them from harm.
"Officers remain on patrol and our staff continue to respond to calls for service as usual."
The force said it had still responded to 999 calls during the recall and that officers had been held on duty while they were given the security briefing, or advised as they started their shifts.
Information that led to the alert is still being assessed, said police, but sources told Sky the threat was "credible".
"We have spoken to police officers from the area who confirmed two facts," said Sky's Sean Dilley.
"The fact that officers have been called back to return to base, and also that they've been asked to watch out for each other when they are actually out on patrol."
The alert comes after four men were charged with terror offences in October, including allegations they carried out "hostile reconnaissance" of Shepherd's Bush police station in London.
They allegedly viewed and kept an image sent via Instagram of two Metropolitan Police officers and two Metropolitan Police community support officers.
