A man in his 30s is in hospital in a critical but stable condition after he was stabbed in the neck with a broken bottle at an upmarket party to promote BlackBerry.
An IBTimes UK reporter was at the BlackBerry BBM party at a nightclub in London's South Bank when violence broke out.
The event, billed as journalists and celebrities-only, was overcrowded and the stabbing is said to have taken place after Jessie J finished her routine.
The star later tweeted: "My thoughts are with the person seriously injured at the blackberry bbm party. so sad. I really hope they are okay."
Police held 450 people in the club for two hours while they launched an investigation. A man of 25 was arrested on suspicion of assault. The victim has not been named.
Witness Karen Jessop said: "I was standing right next to them. They began scuffling and everyone started moving back as you could tell things were about to kick off. But then he hit him with the bottle. It was awful. The blood sprayed all down my arm - it was covered.
"I'm still shaking. I don't really know what to think."
Wretch 32, who had earlier played a set, said: "It's horrible. Absolutely horrible. No-one can really believe this has happened. No-one really seems to know what's going on."
Many celebrity guests, such as Chipmunk and Tinchy Stryder, had left before the stabbing but it is claimed that Jessie J, Wretch 32 and Towie's Mark Wright were among those locked in by police.
A spokesman for BlackBerry said: "Our first concern is the well-being of the injured guest. We are doing all we can to assist police with their enquiries and are unable to provide further comment at this time."
The venue's management would not comment.