The R&B superstar pulled out of last night's awards ceremony after her boyfriend Chris Brown was involved in an alleged assault incident

Singer Chris Brown has handed himself in to Los Angeles police over an alleged assault on an unidentified woman believed to be his girlfriend, the pop star Rihanna.

The couple were due to perform at the Grammy awards last night but they pulled out after Brown, 19, was accused of attacking a woman after an argument in a car. Police have not named the woman involved in the incident, reported to have suffered "visible injuries", but the Los Angeles Times reports witnesses claiming that the woman was Rihanna.

The Los Angeles Times goes on to report that Brown and the unidentified woman were involved in an argument on Sunday evening in Hancock Park, Los Angeles, which escalated to the extent that the police were called. Brown voluntarily attended Wiltshire Division police station later that day.

"The exact charge was criminal threats and it is going to be a criminal threats investigation," said Sgt Bridget Pickett. "There may be domestic violence charges added later, but that will be up to the district attorney's office."

Rihanna, who won a Grammy award in 2007 for her hit single Umbrella, had been due to perform alongside Justin Timberlake at last night's award, while Brown was scheduled to perform his song Forever. Event organisers apologised for Rihanna's absence, while Al Green performed in her place. Publicists for both singers have yet to release a statement concerning the incident.