An "unattended" package has been found near Buckingham Palace in central London, close to the site of Wednesday’s terrorist attack, forcing the Metropolitan Police to deal with it.
What appeared to be the sound of explosions had been heard by people in the area, several British media reported.
The incident was unfolding while mourners gathered for a vigil in London's Trafalgar Square on Thursday evening, following a terrorist attack outside the British Parliament a day earlier.
There have been reports in the media suggesting police carried out a controlled explosion on the package, but a Metropolitan Police spokesman declined to comment on that, Reuters reported.
He said police were investigating the package found near Westminster.
Police later said what had been found was merely an "unattended bag" and lifted security cordons.
On Wednesday, a man later identified by police as Khalid Masood, 52, launched an attack in central London, first ramming a car into people on Westminster Bridge, and then stabbing a police officer by the gates of the Houses of Parliament. Four people were killed and about 40 injured, with Islamic State (IS, formerly ISIS/ISIL) claiming responsibility for the attack.