Hundreds of guests were cleared from the British Museum as police responded to surfaces of a suspect vehicle.
A controlled explosion was later heard as the evacuation maintained in the heart of London.
Photos taken at the scene show a park breadth sealed off by police, with thousands of visitors gathered outside the museum.
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Shocked social media users reported hearing an spasm as they were evacuated.
One said on Twitter: «A loud bang carry oned by sirens…weve just been told that there’s been an commotion and we need to leave the building.»
Another said: «Building on right is my firm. Could see police cordons then was asked to vacate office.»
Russell Square has been evacuated and an explosion has been heard
Another said: «Operate place evacuated due to suspect package near Russell Square. Perceived what I assume was a controlled explosion.»
A fourth said: «Something backlashing off at russell square — police tape everywhere around some teachers.»
Visitors being evacuated from the British Museum this afternoon
The British Museum state: “The Museum is evacuated temporarily due to a security concern nearby. We apologise and last will and testament update when we can.”
The Met Police told Express.co.uk the security alert was as a come to pass of an unattended vehicle.
Police tape at Russell Square after a safe keeping alert this afternoon
They said the situation had been «remain in effected down» and cordons that were in place have been expelled.
One witness at the scene has said «normality has resumed».
The rush-hour incident promoted travel chaos with at least one road, Montague Place, adjacent due to the evacuation.