A coded bomb threat from Irish terrorists sparked major security scares near Buckingham Palace on the eve of the Queen's historic visit to the republic.
A controlled explosion was carried out on a suitcase abandoned outside a hotel and streets around the Mall were shut off for five hours after the warning from Irish republican dissidents.
The caller triggered high alert across central London on telling authorities last night that a bomb was planted in a "non-specific" part of the capital.
Londoners were urged to go about their normal business but to look out for "unusual activity or behaviour" as police took the unusual step of publicising the threat.
The Metropolitan Police is working with City of London Police and British Transport Police, with all officers "advised to be highly vigilant to ensure the safety of London".
The statement said: "A bomb threat warning has been received relating to central London today. The threat is not specific in relation to location or time. Policing operations and contingency planning remain under constant review and a wide range of overt and covert tactics will continue to be used in London.
"At this time Londoners should continue to go about their business as usual but we encourage the public to remain vigilant and report any information about unusual activity or behaviour which may be terrorist-related to the confidential anti-terrorist hotline on 0800 789 321. In the event of an emergency, always dial 999."
The threat level from Irish-related terrorism has not increased and remains at "substantial", meaning that an attack is a strong possibility, according to police.
Roads around the Admiralty Arch area of The Mall were reopened before noon after the area's drains were understood to have been searched. Roads were closed around the ceremonial gateway leading from the corner of Trafalgar Square when an officer spotted "something suspicious" at 4.20am.
A controlled explosion of a suitcase outside a hotel was carried out in nearby Northumberland Avenue as officers attended to suspicious packages.
The alerts came as the largest security operation in the history of the Republic of Ireland swung into action amid fears dissident republicans will try to disrupt the Queen's momentous trip. Armed British police officers will patrol Irish streets alongside thousands of gardai and soldiers to protect the 85-year-old monarch and the Duke of Edinburgh as they travel around the republic.
The threat level for Britain was raised last September from "moderate" to "substantial". This is lower than the overall threat to the UK from international terrorism, which remains at "severe".
A Home Office spokeswoman said: "There is a continuing need for vigilance and the public should report any suspicious activity to the police."
Home Secretary Theresa May warned last year that an attack on Great Britain by Irish-related terrorists was a "strong possibility".