Police officers found an "irresponsible" football match being played at a closed leisure centre in breach of coronavirus restrictions.
The Derbyshire force was called to Etwall Leisure Centre on Sunday afternoon.
It said officers discovered 15 to 20 young people who had forced their way on to the locked outdoor pitch.
The group "made a hasty getaway" when police arrived, leaving an "onslaught of rubbish" and personal belongings.
Parents are being warned they could be fined if their child breaks coronavirus regulations.
Officers from the local safer neighbourhood team arrived at the scene at 13:05 GMT on Sunday.
In a Facebook post, they said: "Officers were horrified to find a group of around 15-20 young people playing football after forcibly making their way on to the locked football pitches.
"After being alerted to police presence, the gathering soon split up and made a hasty getaway.
"This may seem harsh but we need to ensure these rules are being followed.
"A gathering of 20 is simply not acceptable and quite frankly irresponsible in the times we are all living through."
Police said they knew the identities of some of those involved, but have released a photo of personal items left behind in an effort to identify others.
An appeal has also been made to parents, urging them to be aware of their children's whereabouts during coronavirus restrictions.
"It is of the utmost importance that you ensure your children are following the rules and regulations for Covid-19.
"If a child breaches the rules then as their guardian, you will be held responsible for paying the fine," police said.
Also caught breaking coronavirus regulations in Derbyshire over the weekend was a couple who met up in Swarkestone.
One of them had travelled from from Buckinghamshire - nearly 100 miles (160 km) away.
They were found by officers patrolling Lowes Lane - a "well-known antisocial behaviour hotspot" - at about 20:30 GMT on Sunday.
Both were given a £200 fine and sent home.