The man was detained by armed police, after being tasered during the arrest.
The man was arrested after a suspect device was left on a Tube train, forcing police to carry out a controlled explosion at North Greenwich station on the Jubilee Line.
The suspect was taken to a police station and is still in custody. Scotland Yard said the device was due to be forensically examined.
Ali, 30, a witness of the arrest, said the man had been walking “normally” until police arrived.
“The armed police, I think five of them, ran behind him and put him on the floor,” he said.
“They were shouting ‘armed police, don’t struggle’. Everything happened so quick.”
Passengers should expect to see more officers, and even more armed officers, in and around transport hubs.