Police have evacuated Trafalgar Square in central London.
Images and video of the scene show officers moving the crowd behind the police tape.
A police spokesman said many checks were being carried out in the area, which is a short distance from where a concert will be held outside Buckingham Palace later in the day.
He said the incident was believed to involve a suspicious package.
The Metropolitan Police have not yet confirmed the nature of the incident, but have issued the following alert on Twitter: