One man is in critical condition in hospital after a double stabbing at the Wireless music festival in east London on Sunday night.
The man, in his 20s, was stabbed at around 9pm at the Queen Elizabeth Olympic Park in Stratford, police said.
Another man is in non-life threatening condition in hospital.
Police added that no arrests has been made so far and that detectives were continuing their investigations.
According to the Wireless Festival website, US superstars Justin Timberlake and Jay-Z were due to perform on the main stage at the time that police said the incident took place.