Armed police cordoned off an area near Victoria Embankment following reports of a suspicious vehicle.
Metropolitan Police headquarters was also inside the cordon with officers having been called at 3.15pm today.
The disruption also closed Westminster Bridge with members of the public held behind a cordon near Parliament Square.
Met Police tweeted at 3.50pm: “Police are on scene at Victoria Embankment following reports of a suspicious vehicle. Road closures are in place. Updates to follow.”
At 4.30pm they confirmed the vehicle was non-suspicious and roads were being reopened
DevonLive reporter Alex Richards was walking past when the police incident started to unfold.
He said: “There is a huge police presence and the rushed people off the bridge very quickly.
“A cordon is in place across the whole of the bridge and no one is allowed to cross. There’s no indication of what has happened at the moment but police seem to be taking it seriously.”
Some exits at Westminster Tube station were shut. Transport for London advised drivers to avoid the area.
Buses were being diverted and Thames Clippers tweeted: “The River has been closed from Charing Cross Bridge to Westminster Bridge. Boats will not be calling at Westminster Pier or Waterloo Pier.”
Specialist officers are on the scene by the Thames.
It comes days after three bombs stamped with “love from Ireland” were found at Heathrow, London City Airport and Waterloo station.
One of the packages burst into flames when opened at Heathrow and two more had to be made safe by specialist bomb officers.
A terror investigation has been launched and police have issued advice to transport hubs across the capital.