British anti-terror police arrested five people on Monday in pre-dawn raids in Manchester and near London's Heathrow airport, officials said.
The detentions, including a 26-year-old man seized in a hotel near Heathrow, were linked to an alleged overseas threat, media reported, while officials sought to play down the immediate danger.
"This is a complex and ongoing investigation, which has now reached the point where it was necessary to make arrests and speak to a number of people," said Assistant Chief Constable Dave Thompson of Greater Manchester Police.
"Protecting people both at home and abroad is our primary concern, which is why we take such steps," said Thompson, adding that the raids, starting at 4:00 am (0400 GMT), were carried out by the North West Counter Terrorism Unit.
No armed officers were involved in the arrests, which he described as "low-key." No further details were available.
Britain reduced its national terror alert status in July from "severe" to "substantial," its lowest since July 2005 suicide attacks in London which killed 56.
In June 2007 there were failed car bomb attacks in central London and at Glasgow airport in Scotland, days after Prime Minister Gordon Brown succeeded Tony Blair.