The singer is believed to have major contusions to both sides of her face, a split lip, swelling, bruising and a bloody nose, according to TMZ.
The website reports that the singer was heard screaming in Brown's vehicle, prompting a passer-by to call the police.
"Rihanna was clearly very upset by what had happened and I could hear her screaming," an eyewitness told radaronline.com.
"Rihanna was trying to get her point across about something. She sounded like she was crying as she tried to get her point across."
Rihanna was admitted to hospital after allegedly being beaten up by Brown following a row at the pre-Grammys party in Los Angeles.
Police have reportedly dusted Brown's Lamborghini for finger prints after impounding the vehicle yesterday.