Vandals caused £1,225 worth of damage at the newly-opened Saltdean Lido last night (July 9).
Saltdean Lido re-opened last month after a £3 million refurbishment and seven years of hard work by volunteers.
Last night, a statement on the Lido's Facebook page said: "We are sad to report that after a fantastic Sunday down at the pool enjoyed by hundreds, several youths have broken into the site for a swim and have gone on a wrecking spree.
"Flint stones were used to smash toughened glass. Fortunately Sussex Police were quick to respond and several youths have been arrested for criminal damage.
"This is incredibly sad for the volunteers and all of our supporters who have spent the last seven years bringing the pool back from the brink so that generations of young people can experience what it is like to swim in a historic pool.
"We would like to take this opportunity to warn all young people in this area that this has to stop, now. Any unauthorised access to the site after closing hours will be dealt with, with the strictest penalties.
"Swimming without lifeguards is dangerous, and this historic site which must be protected. We are sorry if this sounds a little harsh, but any thoughtless or intentionally criminal activities on this site could bring the whole project into jeopardy.
"We would ask that parents speak to children, youths and young adults about this problem. We restored the pool for them. We would not want a handful of irresponsible trouble makers to ruin Saltdean Lido for the majority of well behaved young people who are excited about having this pool on their doorstep.
"Please help the young people who are doing this understand that this pool must be respected. For, they will be the future custodians of this site."
A Sussex Police spokesperson said: "Police have arrested three teenagers after intruders climbed over a fence into Saltdean Lido and smashed some glass panels surrounding the swimming pool on Sunday night (July 9).
"Two boys aged 16 from Saltdean and Peacehaven and one aged 15 from Saltdean were detained on suspicion of committing criminal damage and are currently in custody.
"Stones were thrown at the panels causing one panel to shatter at about 8.45pm. Around £1,225 worth of damage was caused.
"The open air swimming pool re-opened to the public last month after a seven-year restoration programme."