British authorities have increased security measures following intelligence that terrorists could be planning to attack major transportation hubs.
British transport police cancelled leave and called in extra officers. But officials say the overall threat level has not changed, and there is no suggestion of an imminent attack.
The heightened security steps come after two terrorist plots were linked to Britain in the past few weeks.
A Swedish citizen, who had lived in Britain for the past 10 years, blew himself up in mid-December in Stockholm, wounding two other people.
At the end of December, British police arrested 12 men from around the country, suspected of plotting a terrorist attack in Britain.
Some information for this report was provided by AP, AFP and Reuters.