South East Coast Ambulance Service (SECAmb) said at times it was receiving four or five 999 calls a minute on Saturday (June 29).
It was the hottest day of the year so far, with temperatures recorded at 34 degrees celsius at its peak in the UK.
SECAmb said on social media: “Yesterday was one of the busiest days of the year! At times we were receiving four or even five 999 calls a minute!
“We’re sorry to anyone who waited a long time for a response – our staff are working flat out to get to people asap but some areas remains extremely busy.”
The service also shared an image with the message “we try our best”.
SECAmb covers 3,600 square miles across Brighton and Hove, East Sussex, West Sussex, Kent, Surrey, and North East Hampshire.
It employs more than 3,300 staff working across 110 sites in Kent, Surrey and Sussex, and provides 999 and NHS 111 services.