Police closed Westminster Bridge and Victoria Embankment on Saturday after reports of a suspicious vehicle.
Members of the public were held at a cordon near Parliament Square after police were called at around 3.15pm.
A police spokesman said: "Police are on scene at Victoria Embankment following reports of a suspicious vehicle. Road closures are in place."
He said at around 4.30pm that the vehicle had "been deemed non-suspicious" and roads would soon reopen.
Traffic was being diverted while the cordon remained in place near Westminster Bridge.
Thames Clippers, which runs River Bus, wrote on Twitter: "Westminster pier is currently closed.
"Passengers will not be able to embark or disembark at Westminster Pier."
