A man is in a serious condition in hospital after he was reportedly stabbed in the neck with a broken bottle at a party thrown by smartphone company BlackBerry.
The victim, aged in his 30s, is believed to have been attacked inside Pulse underground nightclub in Southwark, central London, just after midnight.
One man has been arrested in connection with the incident, Scotland Yard said.
Brit-award winning singer Jessie J had earlier performed at the event, hosted by the phone giant to promote its BlackBerry instant messaging service.
The party was attended by journalists, celebrities including rapper Wretch 32 and stars of The Only Way Is Essex and BlackBerry competition winners.
Channel 4 News's technology journalist Benjamin Cohen was at the event and said hundreds of revellers were kept inside the club while police investigated the incident. He also saw a bouncer whose hands and arms were covered in blood following the attack.
Cohen told The Daily Telegraph: "People were crying and quite a lot were being sick, whether from the blood or the alcohol. A lot of people here were teen users of BlackBerry who don't seem to have had a free drink before."
Scotland Yard said a 25-year-old man had been arrested on suspicion of assault and is in custody at a south London police station. The injured man was being treated in an east London hospital, the force said.
Jessie J tweeted about the incident: "My thoughts are with the person who was seriously injured last night at the BlackBerry BBM party. So sad. I really hope they are OK."
BlackBerry said in a statement: "A serious incident occurred at our BlackBerry event. Our first concern is the wellbeing of the injured guest. We are doing all we can to assist police with their inquiries and are unable to provide further comment at this time."