Ravers in France ignored the government's threats to strictly impose an 8 pm COVID curfew, with gatherings reported in Marseille and Brittany.
As many as 2,500 people attended an illegal rave party in Brittany with many "coming from different departments (of France) and abroad", according to the prefecture in Ille-et-Vilaine.
Several law enforcement officers were injured when partygoers threw stones and bottles at them as they tried to break up the event in Lieuron, south of Rennes, and a police vehicle was set on fire, it added.
The authorities then decided to contain the event rather than break it up, and the party continued overnight on Friday before finally ending on Saturday morning (January 2), 36 hours after it began.
France's interior minister said on Saturday a judicial investigation had been opened to identify and prosecute organisers. Gérard Darmanin said 1,200 people were fined for not respecting the curfew, not wearing a mask and illegally taking part in a gathering.
Sound equipment and power generators were confiscated, he added. AFP reported that people leaving were stopped one by one and questioned over drug use. No-one had been able to enter the premises since New Year's Eve, the authorities said.
Some of those who had taken part expressed no regrets. "For a year we haven't been able to do anything," said Antoine, 24, part of a group of five from Brittany who took part.
"We knew what we were risking... We had to party, for a year everything's been blocked," said a 20-year-old woman.
'Strong risk of spreading COVID-19'
Brittany regional health authority said on Saturday that the event "presents a very strong risk of spreading COVID-19" and recommended that party-goers immediately self-isolate at home for seven days and then take a coronavirus test.
"They put their lives in danger, their health, these people must now protect those around them," said Brittany's Prefect, Emmanuel Berthier, calling for a renewed sense of responsibility.
In Marseille, police dispersed an illegal party with 300 people in attendance - over 150 of the revellers were fined for violating the country's curfew, AFP cited police sources as saying.
The event's three presumed organisers were arrested and placed in police custody, Marseille's public prosecutor, Dominique Laurens, said.
France has the fifth-highest number of confirmed COVID-19 cases in the world with its total standing at 2.6 million.
The country has also seen over 64,000 deaths due to the virus.
Meanwhile, "COVID loves a crowd" was the phrase touted in the UK government's advertising campaign emploring citizens to stay at home.
Despite the warnings to "see in the New Year safely at home", some apparently attended parties to ring in 2021.
British media reported mass gatherings in London, with police in the capital saying they shut down an event with around 70 to 100 people in attendance in the borough of Barking and Dagenham.
London is currently under England's tightest COVID-19 restrictions - Tier 4 - which comes with the order to "stay at home", meaning citizens cannot leave or be outside of the place they are living unless they have a reasonable excuse.