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Emergency services descended on a leisure centre after a 66-year-old man collapsed.
Ambulances and a police car were spotted outside the Angel Centre in Tonbridge just before 10am.
The man was resuscitated at the scene by medics, before being taken to hospital in a "very serious condition".
The centre confirmed the gym goer had suffered a cardiac arrest while using the rowing machine.
Staff performed first aid, before using a defibrillator on the patient several times.
On arrival, ambulance crews spent an hour treating the patient, before taking him to Tunbridge Wells Hospital.
A spokesperson from Secamb said: "We were called at 10.20am to reports a man had collapsed at the centre. We sent crews in two ambulances and two cars to the scene.
"Attempts were made to resuscitate him, he was still in a very serious condition and taken to Tunbridge Wells Hospital."
Martin Guyton, CEO of tmactive, which operates the Angel Centre, said: "Our staff undergo regular training to keep their lifesaving skills up-to- date. We are very proud of how they performed yesterday and our thoughts are with the gentleman and his family."
The gym was closed until 1pm as a result of the incident.