Dozens of people gatecrashed Wireless in London, after someone unlatched a metal door into the festival, an online video on Metro shows.
Fans outside the venue are pictured hammering on the doors as security guards try to stop them getting in.
Someone inside the festival then opens the gate, allowing people to rush in.
“This was an isolated incident that was dealt with quickly where no-one was hurt,” a statement made on behalf of festival organisers reads.
“The perpetrators were removed on an otherwise hugely successful first day.
“Organisers of Wireless Festival take security very seriously and work closely with ShowSec Security, local police and other agencies to put procedures in place to ensure the event maintains an enjoyable and secure environment for all its customers.”
ShowSec Security have yet to respond to a request for a comment.
Lethal Bizzle can be heard performing in the background of the video. His half-hour set was scheduled to take place just after 8pm.
The three-day festival is taking place at London’s Finsbury Park.
The Metropolitan Police have been been tweeting from the site, advising people to take care in the hot weather.
Friday’s event also saw performances from Drake, Nero and Duke Dumont. Saturday’s headliner is Avicii, while David Guetta will close the festival on Sunday.