Police have been given special powers after crowds gathered in Cardiff Bay on Friday evening, despite coronavirus restrictions still being in place.
Cardiff council said a significant amount of rubbish had been left by "large groups of people intent on breaking Covid-19 restrictions".
South Wales Police has put a dispersal order in place, giving officers the power to exclude people from the area.
Covid rules state six people from two different households can meet outdoors.
It follows similar scenes outside the Senedd earlier in the week, when three police officers were injured.
Supt Marc Attwell said: "As restrictions have eased, the need for personal responsibility is critical and it is extremely disappointing that a small minority are willing to put themselves, and others, in harm's way.
"Cardiff Bay in particular has attracted large crowds over the past few days.
"The behaviour seen over the past three days has placed additional pressure on not only the police but the local authority as well, and meant that officers have been diverted from other parts of the force."
He added: "We are asking people in groups of six or more who are thinking of attending or organising a gathering at the Senedd over this bank holiday weekend not to do so."
Officers arrested a 20-year-old man for a public order offence, a 22-year-old woman was arrested for being drunk and disorderly and for possession of a Class A substance and a 35-year-old-man was also arrested for possession of a Class A substance.
The council said the trail of litter left a "huge task" for clean-up staff as "bins were left unused and the ground was littered with rubbish".
"Despite the preventative measures put in place by the council, in partnership with South Wales Police, the Welsh government's coronavirus regulations were again broken by a significant number of people illegally gathering in Cardiff Bay," the authority said.
"The rules are clear, six people, from two households are allowed to meet outdoors, maintaining two metre social distancing.
"Breaking these rules significantly increases the chances of Covid-19 cases rising in the city."
Volunteer litter picker Claire Heat, who has been helping with the clean-up since about 06:00 BST, said "there's broken glass everywhere" along with unopened bottles and cans of alcohol.
"I live down the road and you get used to the fact that you know it is going to be awful when you wake up," she said.
"The quicker we can get cleaned up and swept away the better it is for all the people that want to come down and enjoy the bay during the day."
At the scene on Saturday morning: BBC Wales reporter Haf Edwards
Cleaning teams have been here since early this morning. Around me there are beer bottles, cans, crates, shattered glass, and hundreds of nitrous oxide canisters.
Some people have even left picnic blankets and a coat.
The smell of alcohol is lingering in the air.
A lot of the mess was cleared up by 08:30 but one of the cleaning team supervisors tells me they'll be here until later this morning.
Supervisor Tony Tobenas said the area had been "obliterated" by the revellers overnight, which was "demoralising" for the clean-up crew who had been called in to deal with the mess from about 04:30.
"They shouldn't be here today - they should be home," he said.
"We've had to call them in to do this. This is additional [work] and got to be paid for."
He said the mess had become worse since the restrictions allowing wider travel across Wales had been lifted.
"We are supplying bins for them and they just aren't using them. They're throwing things on the floor," he said.
Cardiff council leader Huw Thomas said people who took part should be "deeply ashamed of themselves".
He said police had taken over from council marshals, who had been patrolling the site in the day, after the situation "escalated".
"The clear-up will mean an unnecessary cost to the council and so the taxpayer," he said.
'We're just not ready to party'
Lena Ciric, a microbiologist from University College London, said it was not safe to "party" due to the threat of Covid.
"Not only are there lots of people together in a particular space but, also, they're drinking so they probably won't be wearing masks... inhibitions will be lowered and they'll be closer together," she said.
"We're just not ready to party.
"It's been really difficult for all of us and, you know, I absolutely understand that we want to see our friends and we want to let our hair down because it has been a really, really difficult year.
"But, ultimately, I don't think anywhere in the world, really, we're ready to party."
Welsh Conservative leader Andrew RT Davies said: "This can't continue.
"If people are ignoring the rules on the very steps where they are made then we have a big issue.
"It's time Labour ministers viewed licensed premises as part of the solution, not the problem."
In response to the scenes at Cardiff Bay, Welsh Labour Leader and First Minister Mark Drakeford said: "The pandemic hasn't gone away.
"We need to keep coronavirus rates low so we can keep relaxing restrictions.
"We've come so far since the winter when rates were incredibly high - no one wants to see those sacrifices wasted.
"Stay focused and we can all enjoy the better times ahead."
Rodney Berman, from the Welsh Liberal Democrats, said: "It's disappointing that some people are choosing to ignore the need to social distance and behaving so irresponsibly.
"South Wales Police may need to be more proactive to stop gatherings on such a scale."
A Plaid Cymru spokesperson said: "It is deeply frustrating to see a minority of people disregarding the regulations which are designed to keep us all safe."
"Everyone has their part to play in driving case numbers down. We cannot let the sacrifices we have made over the past months go to waste."