A bomb warning has been received relating to central London, the Metropolitan Police has confirmed.
The force said the threat did not provide specifics about location or time.
The Mall and other central London streets were closed on Monday morning although a link has not been confirmed.
The warning is thought to be related to Irish dissident republicans but the threat level for Northern Ireland-related terrorism has not been changed.
A police statement said officers have been advised to be highly vigilant.
The BBC understands the police received a coded bomb warning.
The police have told the public to continue going about their business but to be vigilant.
BBC security correspondent Gordon Corera said: "My understanding is that this may have come because of a coded warning, which is why there is more concern about it than normal. Threats come in and out all the time.
"But if it was a specific code word warning, that would suggest something more serious and that would be why we have had this alert."
He added it was significant that the threat level from dissident Irish terrorism had not been raised.
"Officials I have spoken to are trying to stress that it is not as if they have concrete intelligence of a specific attack at a particular time, or that would have gone up high in terms of the threat."
The current threat level from international terrorism for the UK is assessed as severe.
The threat level for Northern Ireland-related terrorism is set separately for Northern Ireland and Great Britain. In Northern Ireland it is severe and in Great Britain substantial.
Severe means that a terrorist attack is highly likely, substantial that an attack is a strong possibility