Police are hunting for two men after suspected CS gas was released during a fight on a London Underground train.
The incident took place at 9:13am on Saturday morning on board a Victoria line train, which had stopped at Oxford Circus station in central London.
British Transport Police (BTP) said a number of people were treated at the scene for coughing and shortness of breath.
The BTP has appealed for witnesses and released CCTV images of the two men they would like to speak to.
The force believes the people in the images may have information that could help their investigation.
They added that besides the symptoms shown by the passengers, there were "no further concerns for their health".
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Michael Roberts, who was on board the train with his girlfriend, said he saw two people "looking into the next carriage had seen some sort of a commotion".
Mr Roberts told BBC: "Then two guys stumbled into our carriage and all the people on our carriage moved naturally away from the door.
Kate Howarth, another passenger on-board the train, said: "Pretty scary experience on the Victoria Line this morning.
“Some sort of chemical let off and everyone on the tube started coughing uncontrollably."
"That's when everybody realised we couldn't breathe properly."
In a statement the police force said: “Know them? Gas was released on a tube carriage at Oxford Circus this morning.
“People were treated at the scene for coughing etc.
Further info has shown the gas, believed to be CS spray, was used during a fight on the train. Efforts are ongoing to locate those involved.— BTP London (@BTPLondon) July 20, 2019
Passengers on-board suffered coughing etc. They were treated and there are no further health concerns.
More info: https://t.co/3PbznR51Jr
“No further health concerns. Symptoms suggest the gas was CS gas.”
Transport for London (TfL) has confirmed the substance is not toxic.
A statement on the (TfL) Twitter account read: "We’ve been assured that the substance was not toxic, the BTP are aware of this customer incident & are investigating it fully."
Anyone with information is urged to text the British Transport Police on 61016 quoting ref 171 of 20/07/19.