Revellers who dumped beer and cider cans in Cardiff have been criticised for leaving the area in a mess.
Photographs of workers clearing Roald Dahl Plass, known as The Basin, in Cardiff Bay were posted on Twitter.
Visit Cardiff Bay tweeted "littering is not acceptable" while others blasted people for leaving it behind.
A city council spokesman said he appreciated people wanted to spend more time outside as lockdown eased, but urged them to take rubbish home.
There have also been concern from locals after mass gatherings and littering at places including Ogmore-by-Sea and on Swansea's beaches.
Responding to the Cardiff Bay rubbish, a council spokesman said: "What isn't acceptable is for residents to leave their litter in parks and open spaces, ruining them for everyone else, and expecting someone else to clear up the mess they've left behind.
"The pandemic is not over and the council is using the resources available to us to ensure we can deliver the best possible services for the residents of Cardiff.
"People need to take responsibility for their own waste. If the litter bin is full, please take your litter home with you to dispose of correctly."
In a series of tweets, the council also called littering "a zero tolerance offence" that can bring a £100 fine.
It said its enforcement team was aware of the situation and larger bins are due to be placed at Cardiff Bay next week to encourage people to dispose of rubbish properly.
In Wales, it is illegal to gather in large groups and people are advised not to travel outside of their local area because of the coronavirus pandemic. Social distancing measures also remain in place.
But people gathered at a number of locations on Thursday, including Ogmore-by-Sea, Vale of Glamorgan, where there were reports of a large brawl and people had to clean up the rubbish left behind.
People who live near the beaches of Rotherslade, Langland and Caswell in Swansea said they had been forced to pick up "huge amounts" of litter every morning following almost-nightly parties over the last week.
Eve Haynes, who lives at Rotherslade Beach, near Mumbles, said fireworks were lit from the road and aimed at the beach on Thursday night.
"They came straight past us and they erupted over people and around people that were on the beach. There were children, there were young people and there were families, and it was just really quite scary," she said.
Natasha Jenkins has helped pick up litter on Langland Bay every morning this week and said: "24 hours after we had cleaned the beach, it looked even worse the next morning".
Chris Knight, who lives in the area, said the situation with raves at Langland Bay were getting so bad, he was concerned violence could erupt.
"The police need to do more. It's so disappointing. There's a lack of education but also a lack of consequence. If they get away with it they'll keep doing it," he said.
Police in Swansea have already issued three dispersal orders in two weeks to tackle anti-social behaviour.
South Wales Police said: "Extra patrols will be taking place in the Langland Bay area and we would like to remind people that Covid-19 legislation remains in place and anti-social behaviour will not be tolerated."