Republican dissidents threaten to bomb London
Republican dissidents have issued a bomb threat for central London, Scotland Yard confirmed.
Security was heightened after a "non-specific" threat was issued to authorities on Sunday night.
Amid increased police activity across London, areas around the Mall were cordoned off for several hours.
A Scotland Yard spokesman said: "A bomb threat warning has been received relating to central London today. The threat is not specific in relation to location or time."
Londoners were urged to go about their business as usual but to look out for "unusual activity or behaviour" in a statement issued by police.
Scotland Yard officers were working with the City of London Police and British Transport Police, with all officers "advised to be highly vigilant to ensure the safety of London".
The threat level from Irish-related terrorism has not increased and remains at substantial, meaning that an attack is a strong possibility, police added. The threat level 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.
Alerts near Buckingham Palace came on the eve of the Queen's historic four-day visit to the Republic of Ireland. Roads around the Admiralty Arch area of the Mall were reopened before noon after a security scare in the early hours.
A painstaking search was launched and roads were closed around the ceremonial gateway that leads from the corner of Trafalgar Square after an officer spotted "something suspicious" at 4.20am. There was also understood to have been a controlled explosion of a suitcase in nearby Northumberland Avenue as officers swooped on suspicious packages.
A UK Home Office spokeswoman said: "The threat level to Great Britain from Northern Irish-related terrorism remains at substantial, which means that an attack is a strong possibility. There is a continuing need for vigilance and the public should report any suspicious activity to the police."