People could be prosecuted if they take part in a planned water fight in Battersea Park in London.
Wandsworth Council said organisers were inviting people through Facebook to participate in the event in late July.
Police officers with dogs will patrol the park entrances and stop suspects, who could face up to a £200 fine.
The warning comes after a similarly organised water fight in Hyde Park resulted in disorder and arrests.
Three people were arrested after some 1,500 people got involved in the water fight on 4 June, which continued for eight hours and resulted in the closure of Oxford Street to traffic.
The council said the use of water pistols, fighting, brawling, using offensive language or causing annoyance to others in the park was prohibited. It has also warned people online.
Councillor Sarah McDermott, the environment spokesman, said: "We do not intend to sit idly by and allow the park to be used for a mass water fight.
"Many people may be thinking of attending because they believe it will be a harmless and amusing day in the park.
"Unfortunately experience elsewhere has shown that whenever these events are organised they normally descend quite rapidly into violence, mayhem and misery for the innocent people caught up in it.
"My advice to anyone thinking of coming to Battersea Park for a water fight is stay away."