Six fire engines, two fire rescue units and around 50 firefighters and specialist officers were called to an incident at Parsons Green Tube station.
Director of Operations Tom George said:
“The Brigade was called at 0821 and firefighters were on the scene within three minutes. Fire crews assisted the London Ambulance Service in treating casualties on the affected train.
“Firefighters also helped evacuate 253 people from a train not involved in the incident. We continue to support the Metropolitan Police and other services at Parsons Green.”
Fire crews from Fulham, Wandsworth, Chelsea, Hammersmith and other surrounding fire stations were in attendance.
A Metropolitan Police Service spokesperson said: "Officers from the Met's Counter Terrorism Command are investigating after an incident on a London tube train this morning.
"Police were called at approximately 08:20hrs on Friday, 15 September to Parsons Green Underground Station following reports of a fire on the train.
"Deputy Assistant Commissioner Neil Basu, the senior national coordinator for CT policing, has declared it a terrorist incident.
"Officers from the Metropolitan Police Service and British Transport Police attended the scene, along with colleagues from the London Fire Brigade and London Ambulance Service.
"At present we are aware of a number of people who have suffered injuries.
"It is too early to confirm the cause of the fire, which will be subject to the investigation that is now underway by the Met's Counter Terrorism Command.
"The station remains cordoned off and we are advising people to avoid the area.
"Anyone who has information that would assist detectives is urged to contact the hotline 0800 789 321; if it's an emergency always call 999.
"Anyone with images from the scene is asked to upload them to www.ukpoliceimageappeal.co.uk
"We urge the public to remain calm but alert and if you have any concerns, see or hear anything suspicious then contact police."
Natasha Wills, Assistant Director of Operations at London Ambulance Service said:
“We can confirm we have taken 19 patients to hospital following the incident at Parsons Green underground station.
"We were called at 8:20am today, with the first ambulance crew arriving within five minutes.
“We have sent a number of resources to the scene including ambulance crews, officers and our Hazardous Area Response Team.
“We are working closely with our colleagues from the Metropolitan Police, British Transport Police and London Fire Brigade at the scene; ensuring patients receive the medical help they need as quickly as possible.
“As we are very busy dealing with this incident, we would ask the public to only call us in a genuine emergency.”