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.
The BBC understands the police received a coded bomb warning.
The Met Police statement said: “All officers have been advised to be highly vigilant to ensure the safety of London.”
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”
End Quote Gordon Corera BBC security correspondent
It 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.”
A Home Office spokesman said the UK faced a “real and serious threat from terrorism”.
“The overall threat level from international terrorism remains at severe which means that an attack is highly likely.
“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.”
Earlier on Monday, The Mall and Carlton House Terrace were closed after a disturbed manhole cover sparked an alert.
The Metropolitan Police have confirmed they are investigating a break-in at Carlton Gardens but could not confirm whether or not the incident was linked to the closure.
The building involved is believed to be 2 Carlton Gardens, which houses the Institute for Government think tank.
That is adjacent to the foreign secretary’s residence, where William Hague is believed to have been at the time.
Source: BBC News