A Tube station had to be evacuated evacuated after a smoke bomb was set off.
The device went off at around 3.20pm and the station was closed for 40 minutes while the smoke cleared, the BBC reported.
One person has been arrested in connection with what happened, British Transport Police said.
One witness who filmed what happened told HuffPost UK the scene as "madness".
Allie Elwell filmed a group of football fans wearing Crystal Palace colours after they had come down the escalators and into the station, followed by police officers.
She told HuffPost UK: "It was madness. There were fans singing and cheering down the tunnel. We turned the corner and could smell smoke.
"The station filled with smoke, cops everywhere. Fans seemed to just ignore it all, still singing and banging on the side of the escalator.
"Not soon after, there was announcement on the PA to evacuate due to an emergency. We jumped on one of the last tubes [to leave before the evacuation]."
Other witnesses described what they saw on Twitter.
According to the BBC, the station was reopened by 4pm.