A suspected CS gas attack has occurred on a London Underground train.
It happened at 09:13 BST on board a Victoria line Tube train at Oxford Circus in central London.
British Transport Police said a number of people were treated at the scene by paramedics for coughing and lack of breath – but there were “no further concerns for their health”.
The force has appealed for witnesses and released CCTV images of two people they want to speak to.
Passenger Michael Roberts was with his girlfriend on their way to Oxford when he said he saw two people “looking into the next carriage had seen some sort of a commotion”.
“Then two guys stumbled into our carriage and all the people on our carriage moved naturally away from the door.
“That’s when everybody realised we couldn’t breathe properly.”
The 26-year-old said at first he thought the men were drunk “because they were trying to frantically get into the carriage”.
Mr Roberts said he had his T-shirt over his mouth because he could not breathe and his girlfriend was “spluttering”.
He said the effects of the gas lasted about an hour and described the ordeal “pretty frightening”.
A Transport for London spokeswoman said the train had been taken out of service and to a depot for quarantine.
Oxford Circus, situated on the junction of Oxford Street and Regent Street, serves the Central, Victoria and Bakerloo lines.
CS gas, also known as tear gas, can cause a burning sensation around the eyes and difficulty in breathing.