Around 3,000 revellers aged 18 to 20 danced to pounding music in a warehouse on the docks, in close contact with each other and without wearing masks, according to a Reuters witness.
Attendees at the evening event had to test negative for COVID-19 within 24 hours of the event.
“We’re excited. We’re all on the verge of tears ready to go in,” a student called Josh told the BBC while queuing outside the venue.
The two-day event is part of a pilot scheme to see how more venues can be reopened safely. It also included a soccer cup final attended by 8,000 people on Sunday at London’s Wembley stadium.
Researchers will accumulate data from these events to see how approaches to social distancing and ventilation affect the spread of the virus.
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Non-essential retailers in England reopened on April 12 along with pubs and restaurants operating outdoors, and from May 17 restrictions will be lifted further to include indoor hospitality, performances and sporting events.
Prime Minister Boris Johnson hopes to lift the lockdown entirely in June.
While Britain has rolled out COVID-19 vaccines much faster than most of its European peers, it has recorded more than 127,000 COVID-19 deaths, the fifth-highest death toll globally.