A man with a knife has been arrested near Buckingham Palace in an incident Scotland Yard said was not believed to be terror related.
An eyewitness said he saw a man surrounded by officers and handcuffed as he sat on the ground in The Mall at around 10.40am on Wednesday.
A police van arrived with lights flashing and drove him away.
The arrest happened shortly before the Queen was driven from the palace for an engagement with the Duke of Edinburgh at St Paul's Cathedral.
They are attending a service to mark the centenary of the Order of the British Empire.
A spokeswoman for the Metropolitan Police said no-one was injured in the incident.