The fracas occurred on Tuesday evening when Bieber and Disney starlet Selena Gomez left an Italian restaurant where they had celebrated his 17th birthday.
A grim Bieber made the spontaneous gesture after he took the wheel of their vehicle and Gomez shielded her face. It was a rare outburst by the pop sensation, who is publicly unflappable and upbeat in the toughest of circumstances.
"I'm sorry," he wrote on Twitter Wednesday. "It's not always easy but I know better than to react in anger."
He added that life has its "ups and downs" and that he would make mistakes, but he would learn from them and try to do the right thing.
On the positive side, he told his 7.8 million followers that they had raised more than $40,000 for a charity that provides clean water in poor nations. Separately, TV talk show host Ellen DeGeneres said she sold a lock of Bieber's hair at auction for $40,668 with proceeds going to an animal charity.
(Reporting by Dean Goodman; Editing by Bob Tourtellotte)
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