Police evacuated Trafalgar Square on Saturday morning and advised people not to travel to the central London area.
However, the Metropolitan police later confirmed that an “incident” had since been “concluded” and the area would reopen in “due course”.
A police spokesperson said the incident was thought to have involved a suspicious item, The Telegraph newspaper reported.
A number of people on social media also said they had heard a loud bang, with unconfirmed reports that police had carried out a controlled explosion on a car.
There were pictures on social media of police asking people to leave nearby coffee shops and hotels on Saturday morning.
It comes as crowds of people have flocked to central London this weekend to celebrate the Platinum Jubilee bank holiday.
Police later confirmed the incident was not terrorism related.
The London Fire Brigade attended the scene while the ambulance service said it was aware of the police incident but had not been called, according to a report by The Telegraph newspaper.
An open air concert is set to take place outside Buckingham Palace later this evening, with performances from Queen, Alicia Keys, and a record performance from Sir Elton John.