Brighton beach was littered with laughing gas canisters and balloons by Pride revellers, say volunteers cleaning up after the event.
Up to 1,000 people have been clearing large amounts of bottles, cans, glass and disposable barbecues.
But they said they had never seen so many nitrous oxide canisters in one place.
By 10:00 BST more than 150 bags of rubbish had already been collected, organisers Ocean’s 8 said.
Environmental activist and blogger Clare Osborn, of Clare Talks Rubbish, is one of Ocean’s 8 and said: “We sound like the fun police, but people really need to find more sustainable ways to have fun.
“Every one of these canisters comes with a balloon, and they are so incredibly dangerous and deadly to wildlife, which can mistake the bits of balloon for food.”
The gas – nitrous oxide – is the second most commonly used recreational drug in England and Wales after cannabis.
But the Royal College of Nursing said many people remained ignorant of the risks, which can include breathing difficulties, dangerously-increased heart rate, burns, and death.
Three time slots have been created throughout the day for people to go to the beach and help collect the rubbish.
Core volunteers then sort it into what can be recycled with TerraCycle, and the remaining waste which is being collected by Brighton and Hove City Council’s Cityclean service.
Amy Gibson, another member of Ocean’s 8 who organises regular Pier 2 Pier silent disco beach clean, said she has never seen so much rubbish on the beach.
“You couldn’t even see the beach in places.
“Normally we find around 20 bags’ worth of rubbish after a weekend or an event, but we’ve collected 10 times that amount in the first two hours of today’s clean.
“It’s Pride so we expected the glitter, the feather boas and the parts of people’s costumes, but I can’t believe how many noz canisters there are.”
Legislation introduced in 2016 made it illegal to sell the gas for psychoactive purposes.
But enforcing the legislation has proved difficult because nitrous oxide is used in food products like whipped cream, and medicine.
Brighton Pride does not officially hold any events on the beach, but it said it recognised the impact tourism had, so it had sponsored the clean-up effort.
About a quarter of a million people visit Brighton for the city’s annual Pride weekend.
Kylie Minogue headlined the music event, supported by surprise guest Emeli Sande, and more than 100 floats took part in the parade.
The event comes during the 50th anniversary year of a riot which started in a gay bar in New York, sparked by police brutality.