Justin Timberlake and Cameron Diaz, who announced last week that they'd split up, got into a heated argument at Prince's Golden Globes afterparty early Tuesday morning.

The exes, who'd chatted politely during the awards ceremony and even shared laughs at InStyle's annual afterparty, hit the rough patch in their officially amicable breakup at a wee-hours bash hosted by the singer at the Beverly Wilshire hotel.

According to witnesses, Timberlake was chatting with actress Jessica Biel when Diaz approached. Biel stepped away, and the former couple had a serious exchange.

Diaz walked away, Timberlake followed, and the two were soon spotted engaged in an intense 40-minute face-off in a side room.

Diaz reemerged looking composed, but not before Timberlake held his head in his hands and, says a witness, "slammed his fist (down) into a cabinet." (The New York Post's Page Six also reports on their Globes run-ins here.)

Like the old song says, breaking up is hard to do.