A controlled explosion was carried out on a suspicious package found near Horse Guards Parade, witnesses say.
Police confirmed they were called to a suspect item discovered in Birdcage Walk, not far from Buckingham Palace.
Scotland Yard would not confirm whether a controlled explosion had taken place but said the incident had been stood down.
However, people nearby said a blast had been carried out.
Witness Paul Abernethey tweeted: “Down in Westminster, just heard what sounded like an explosion.
“Not sure what happened though. Police said they were dealing w/ new incident.”
Mr Abernethey said police officers and a helicopter quickly arrived on the scene.
BBC producer Pia Talbot said a controlled explosion had been carried out near where the corporation’s crew was filming.
She said officers had described the incident as “routine”.
It came as London was on high alert after Wednesday’s terror attack on Parliament.