A boat party full of people were fined after 40 turned up and paid £30 each to attend.
The incident was listed as one of the worst breaches of lockdown rules as police confirmed that 45,000 fines had been issued across the UK.
Martin Hewitt, chair of National Police Chiefs' Council, told a Downing Street briefing his officers will no longer "waste time" reasoning with the worst violators.
Mr Hewitt outlined a series of recent examples of "irresponsible behaviour" and said officers will punish people who have "no regard" for the safety of others.
Some of the behaviour officers have to deal with included:
- A boat party in Hertfordshire with more than 40 people where people paid £30-a-head to attend
- A minibus full of people from different households travelling from Cheltenham into Wales for a walk.
- A party in Surrey which organiser claimed was a business event
- A man who was advertising a rave on social media
He said: "Organising parties or other large gatherings is dangerous, selfish and totally irresponsible in light of the current threat that we face.
"Organisers will be fined. But so too will the people who choose to attend."
But he also warned that people would be fined for failures closer to home.
He said: "Not wearing a face covering on a bus or a train is dangerous.
"It risks the lives of other travellers including those critical workers who must continue to use public transport to do their important work.
"So on those systems, unless you are exempt, you can expect a fine."
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Home Secretary Priti Patel said the "horrifying" death toll underlined the need for people to follow the coronavirus rules.
Ms Patel said: "My message today to anyone refusing to do the right thing is simple: if you do not play your part our selfless police officers - who are out there risking their own lives every day to keep us safe - they will enforce the regulations.
"And I will back them to do so, to protect our NHS and to save lives."
She said "far too often" police officers were risking their health and lives by "coming into close contact with people, including those who deny the very existence of coronavirus, to keep us all safe".
