People were stopped from entering or leaving the BBC's Broadcasting House on Monday as police investigated what they said was a suspicious vehicle.
A bomb disposal robot approached the orange van and a number of controlled explosions were carried out.
Roads were closed around the area in central London and BBC staff were advised to stay away from windows.
Police cut open the doors of the van, which was found to contain cardboard boxes and a motor bike.
All entrances and exits of New Broadcasting House were closed at around 16:50 BST while police carried out their investigations on nearby Duchess Street, which is to the rear of the BBC's headquarters.
It left some staff unable to return to work - and others unable to leave. The building reopened at 18:30.
Scotland Yard said all roads were re-opening.
#W1 incident stood down and all roads to re-open... (We were called at 3.15pm to a report of a suspect vehicle on Portland Place #Westminster. The incident has now been stood down and police cordons are being removed. Thanks for your patience as officers worked at the location)— Metropolitan Police (@metpoliceuk) September 3, 2018
An AI conference was also taking place at the building at the time.
One participant tweeted it had been a "surreal" afternoon.
While we are on bomb threat lockdown during the AI conference at BBC Broadcasting house, and a robot is outside blowing things up, some of the BBC helpers are doing Tai Chi. Well, this just got more surreal.— Dr Beth Singler (@BVLSingler) September 3, 2018