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Trafalgar Square is believed to have been evacuated following the discovery of a suspicious vehicle.
The central London area is closed with images showing a large police presence and cordons in place.
Several people on social media reported hearing the sound of an “explosion” or an explosion around 11 a.m.
Westminster police said the incident was over and officers remained at the scene. However, they did not disclose details of what happened.
The force said on Twitter: “Officers are currently at #TrafalgarSquare where they evacuated the site. Please do not travel to the area.
Police have evacuated Trafalgar Square
They later added: “Police are still on the scene. The incident is over and the square will be reopened in due course.”
A journalist at the scene said a controlled explosion was used to deal with the suspicious vehicle and urged people not to panic.
Crystal Goomansingh of Global News tweeted: “Controlled explosion near Trafalgar Square. Police say officers dealt with a suspicious vehicle. Please don’t be frightened because you heard a blow,” says an official of the Met at work.
The London Fire Brigade is also at the scene.
Tourist Sarah Churchill, who was in Trafalgar Square at the time with her 11-year-old son Noah, said she heard the sound of a “bomb” firing “without warning.”
A controlled explosion is believed to have taken place after a suspicious car was found (Image: @ summee777 / Twitter)
He told the Daily Mirror: “There were a lot of police, they were about to walk down the square when we got to the corner we saw that it was cordoned off.
“We were talking to a police officer when the bomb exploded without warning.
“We had been told it was a suspicious package, but it was not a controlled explosion. No one told us it was about to explode.”
Sarah, 38, said the sound of the bomb “shook her” and was “intensely loud and fast.”
He said his son was terrified because the noise caused a crowd of about 150 people to start fleeing.
Images on social media also show people being evacuated from a nearby hotel due to a “suspicious car that was not guarded.”
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He arrives before tonight’s Platinum Jubilee concert in front of Buckingham Palace.
More than 20,000 people will attend the open-air ceremony to honor the Queen’s 70th birthday.
The event will be opened by the rock group Queen, with other stars such as Diana Ross, Alicia Keys and Sam Ryder from Eurovision.
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