A man is in a serious condition in hospital after he was reportedly stabbed in the neck with a broken bottle at a BlackBerry party where Jessie J had been performing.
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.
The Brit-award winning singer had earlier sang at the event, hosted by the phone giant to promote its BlackBerry instant messaging service.
The shaken performer posted her condolences on Twitter today.
The party was attended by journalists, celebrities including rapper Wretch 32, Lauren Goodger, Lydia Bright and Joey Essex of The Only Way Is Essex.
BlackBerry competition winners also mingled with X-Factor judge Tulisa Contostavlos's ex-boyfriend N-Dubz rapper Fazer, Nicola McLean, Lizzie Cundy and Jessica Jane Clement.
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: "Everyone tried to leave at the same time but the organisers weren't geared up for it and there was a huge rush to get to the cloakroom.
"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."
Photographs posted online following the incident seemed to show pools of blood on the floor near the clubs bar.
A Scotland Yard spokesman said: "At around 12.07am the ambulance service called police regarding a man injured in a club off Southwark Street.
"He is thought to be in his 30s and has been taken to hospital in a serious condition.
"The victim is thought to have been injured with a bottle. One man has been arrested and is in custody."
Scotland Yard said the 25-year-old suspect 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.