Knut - Berlin's world-famous polar bear - who rose to international stardom as a cuddly cub hand-raised by zookeepers, died suddenly today, a zoo official said.

The bear died alone in his compound without warning, bear keeper Heiner Kloes said.

"It was a completely normal day. He was with the female bears before, who had just been shut away," Kloes said. "Then Knut strolled around the enclosure, went into the water, had a short spasm and died."

A post-mortem examination will be conducted on Monday to try to pinpoint the cause of death, he said.

Between 600 and 700 people were at Knut's compound and saw the four-year-old bear die, German news agency DAPD reported.

One visitor said she watched Knut lying on the surface of the water motionless with only his back showing for 10 minutes until zookeepers came and fenced off the compound.

"Everybody was asking, 'What's going on, why is Knut not moving?'," said Camilla Verde, a 30-year-old Italian who lives in Berlin.

"All the zoo keepers who put up the fences were so very sad. One of them said, 'He was our baby'," she said.

As the news of Knut's death spread through the city, more Knut fans showed up at the zoo, assembling in front of the bear compound to mourn his loss.

Berlin Mayor Klaus Wowereit said Knut's death was "awful."

"We all held him so dear," Wowereit told daily newspaper B.Z. "He was the star of the Berlin zoos."

The polar bear rose to global fame after he was rejected by his mother when he was born in captivity on December 5, 2006. The fluffy cub was shown to the public 15 weeks later, and attendance at the zoo has roughly doubled since, officials said.

"Knutmania" led to a 2007 Vanity Fair cover with actor Leonardo DiCaprio shot by famed photographer Annie Leibovitz, a film and cuddly likenesses.

Though the zoo has never released exact numbers, Knut merchandise including postcards, key rings, sweets and stuffed Knuts have brought in hundreds of thousands of euros.