Irish republican dissidents have issued a bomb threat for central London, Scotland Yard confirmed today.
Security was heightened today after a "non-specific" threat was issued to authorities last night.
The alert coincides with the first visit of Queen Elizabeth to the Republic of Ireland tomorrow.
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 Metropolitan Police statement said: "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, 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.