After taking the stage Saturday, the troubled singer climbed down into the pit and scuffled briefly with a reveler.
It was unclear what sparked the altercation but witnesses say a fan tried to grab Winehouse.
Winehouse sang for about an hour in front of an estimated crowd of 80,000.
She shocked fans last week by performing at a special birthday concert for Nelson Mandela.
The performance came just after she was hospitalized. Her father says she developed emphysema from smoking cigarettes and crack cocaine, although her spokeswoman has said Winehouse only has pre-emphysema symptoms.