The referee David Coote called the match to a halt after being alerted to the emergency in the crowd by Chelsea full-back Marcos Alonso and matchday stewards.
Paramedics rushed to help the person in question while both sets of players returned to the dressing room and waited for definitive news before deciding whether to continue the match.
After 20 minutes, the supporter was carried out of the stadium, having been stabilised by the medical staff, and was then taken to hospital.
The stadium announcer confirmed to those in attendance that the supporter had been stabilised before the players returned and continued the match.
Watford tweeted: “The players have returned to the field in preparation of play resuming.
“Our thoughts are with the fan – who had a cardiac arrest but has now been stabilised – and all those affected.
“Thank you to the medical staff, players and fans for their quick response.”
A Chelsea statement read: “The thoughts of everyone at Chelsea Football Club are with the fan and all those affected.
“Our thanks to the medical staff here at Vicarage Road for their quick response.”
Register for free to continue reading
Registration is a free and easy way to support our truly independent journalism
By registering, you will also enjoy limited access to Premium articles, exclusive newsletters, commenting, and virtual events with our leading journalists
Already have an account? sign in
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies