Grammy day was supposed to be a glittery affair for R&B's reigning It couple, Chris Brown and Rihanna. But things went horribly amiss.

About the time Brown was scheduled to be singing "Forever" onstage at Los Angeles' Staples Center, he was instead detained by police, being questioned for allegedly roughing up Rihanna.

Brown, 19, turned himself in at the LAPD's Wilshire station around 6:30 p.m. and was booked on suspicion of making criminal threats, a charge that carries a maximum penalty of four years. The case will now be sent to the district attorney's office and further charges could be filed. He was released from jail after posting $50,000 bail.

"He was very professional and didn't appear to have injuries," said Sgt. Bridget Pickett, adding, "He's a good looking young man."

While Brown appeared unscathed, according to sources the alleged victim was Rihanna, 20, and that she suffered visible bruises on her face during an early morning confrontation. Both in turn nixed scheduled Grammy appearances.

A spokeswoman for Brown's label declined comment and calls to his attorney, Mark Geragos, weren't immediately returned.

Despite sources saying that Rihanna had been hospitalized, her publicist, Amanda Silverman, said the "Disturbia" singer "is well. Thank you for concern and support."

Per department policy, police declined to identify the alleged victim in the incident, which occurred about 12:30 a.m. in the Hancock Park area, apparently as the two were heading home from Clive Davis' annual pre-Grammy bash, where they had been all smiles.

In a statement, police say Brown and his companion "became involved in an argument. After stopping his car, Brown and the woman got out and the argument escalated. The woman suffered visible injuries and identified Brown as her attacker."

By the time officers arrived on the scene, Brown had split.

Rihanna, whose full name is Robyn Rihanna Fenty, decided against singing a medley of "Disturbia" and "Live Your Life" at tonight's awards. Likewise, Brown scrapped his scheduled performance of "Forever."

"We're sorry she is unable to join us this evening," said the Recording Academy. A virtually identical statement was issued for Brown's absence.

Brown and Rihanna were up for dueling Pop Collaboration With Vocals awards, Brown for his duet with Jordin Sparks on "No Air," Rihanna with Maroon 5 for "If I Never See Your Face Again." Brown was also contending for Male R&B Vocal Performance for "Take You Down." Neither Brown nor Rihanna won anything.

After the ceremony, Recording Academy President Neil Portnow said that he first learned of the situation "pretty late in the afternoon," about three hours before showtime.

"Musicians are no different than anybody else. I'm not judging. I'm sorriest that they weren't there for their moments on stage," he said, adding that he's never had to deal with replacing two segments at the last minute.

"I'm hoping that [the awards are] the story and should not be eclipsed by this part of business," Portnow said. "I think the story is...the music that went on stage."

Backstage, Anthony Hamilton, winner of the Traditional R&B Vocal Grammy with Al Green, said his pal was pressed into emergency service "about an hour and a half before the preshow."

Green crooned "Let's Stay Together" with assists from Justin Timberlake, Keith Urban and Boyz II Men.

Asked if old-pro Green really was able to take the stage on such short notice, Hamilton suggested the R&B icon was practicing last night.

Said Hamilton, "He knew something could happen."