Around three thousand people in the UK city of Liverpool danced, drank, and listened to music without masks or social distancing.
The event on Friday was one of a series of pilots sponsored by the UK government to test whether covid-19 restrictions can be eased without sparking new outbreaks.
"Feels dead weird to be back around so many people and, like, without masks, without social distancing. Honestly, it feels amazing," said one young clubber after the event.
"It's really surreal," added another. "It feels like, half of me feels like it's not been any time at all and the rest feels like it's been forever. So it's really good to be out again."
The event was closely monitored by public health officials. Everyone who went had to be tested before and after. Researchers were also planning to study air quality and movement inside the venue.
A second clubbing event was due to be held in the same venue on Saturday, and 5,000 fans will attend a concert headlined by the band Blossoms at Liverpool's Sefton Park on Sunday.
The test events will culminate at Wembley Stadium on May 15, where a one-quarter capacity crowd of 21,000 will watch the FA Cup soccer final.