Hundreds of fans of Knut the polar bear flocked to his zoo enclosure Sunday to mourn the sudden death of the celebrity who burst into the limelight as a cuddly, fluffy cub hand-fed by his keeper.

Visitors lay down flowers and condolence messages at Knut's enclosure at the Berlin Zoo on March 20, 2011 in Berlin, Germany.

The beloved four-year-old died Saturday afternoon in front of hundreds of visitors, taking keepers, animal experts and fans by surprise.