Officers in the West Midlands broke up some large New Year's Eve parties after finding some "shocking Covid breaches", the area's police chief said.
Chief Constable Sir Dave Thompson, who spent time with the force's Operational Support Unit on Thursday, said some people had ignored new tier four rules.
However, he tweeted the "majority have done the right thing".
The force issued 20 fixed penalty notices and said about 50 party goers "ran from an address" in Wednesbury.
Officers went to the premises in Warner Road just before 23:30 GMT and the homeowner was told he would be fined, a force spokesman said.
"We will review CCTV and body-worn video to determine if there are grounds to issue a £10,000 fine for a deliberate and reckless breach of Covid-19 rules," he added.
Earlier, the organiser of a party in William Street, in Birmingham city centre, was fined and others told to leave the flat.
"We believe up to 30 people may have been at the address with some running off having climbed through a window," police said.
Officers were also called to a drive-through Shisha lounge in Digbeth where a significant number of vehicles and people had gathered, said police, adding the organiser was told he would "receive a large fine".
Assistant Chief Constable Matt Ward said the West Midlands force received more than 700 calls about suspected breaches on Thursday night.
"The vast majority were low-level breaches and, on the whole, officers spoke to people they encountered and moved them on," he said.
Most of the West Midlands is now under tier four restrictions where people have been told to stay at home to avoid spreading Covid-19 as cases across the UK continue to rise.