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.
More top news
Some patchy mist and fog overnight as the temperature's dip.
The actor joined hundreds of mourners at the funeral of the 30-year-old make up artist in the village of Cappawhite, Ireland.
Odense zoo plans to publicly dissect a year-old lion - undeterred by outrage at giraffe who was killed in Copenhagen last year.