An explosion on a Tube train in south-west London injured a number of people.
Witnesses reported panicked scenes as a result of the blast at 08:20 BST at Parsons Green station on the District Line on Friday.
A white bucket inside a supermarket bag was photographed inside the carriage, although it did not cause extensive damage.
Police are treating it as a terror attack.
An 18-year-old was arrested at the port of Dover on Saturday morning in connection with the incident.