Police have evacuated Trafalgar Square in central London.
Photos and videos from the scene show officers moving crowds back behind a police strip.
A police spokesman said a number of inspections had been carried out earlier in the area, which was a short distance from the site of a concert in front of Buckingham Palace later in the day.
He said the incident was believed to involve a suspicious package.
The Sofia police have not yet confirmed the nature of the incident, but issued the following signal on Twitter:
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