BERLIN – Berlin's beloved polar bear Knut, an international star who as a cuddly, fluffy cub graced magazine covers, movies and merchandise, died Saturday. His death at the young age of four took zookeepers and animal experts by surprise.

Staff stand near the polar bear enclosure in Berlin zoo, March 19, 2011. [Photo/Agencies]

The celebrity bear died suddenly in his compound at the Berlin Zoo on Saturday afternoon, bear keeper Heiner Kloes told The Associated Press. He waded into the water in his enclosure before having a short spasm and then dying in front of hundreds of zoo visitors.

While the life expectancy of polar bear in the wild is between 15 and 20 years, animals in captivity can live even longer because they are not exposed to hunger, thirst or infections. A postmortem will be conducted on Monday to try to pinpoint the cause of death, Kloes said.

"He certainly did not die of old age," Thomas Pietsch from the Vier Pfoten group for the prevention of cruelty to animals told German news agency DAPD.

His death was met by an immediate outpouring of sorrow. 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 called Knut's death "awful," telling the B.Z. daily, "we all held him so dear."

"He was the star of the Berlin zoos," he said.

Rejected by his mother at birth on December 5, 2006, along with his twin brother, who only survived a couple of days, Knut first attracted attention when his main caregiver, Thomas Doerflein, camped out at the zoo to give the button-eyed cub his bottle every two hours. Doerflein cuddled and played with him at daily public appearances to the delight of thousands of people who came to watch.

Fan clubs sprung up across the globe, including in Japan, the United States and Germany. Fans followed his every move, including his weight battle — he had a weakness for croissants — or plans to move to a different zoo.

"Knutmania" led to a 2007 Vanity Fair cover with actor Leonardo DiCaprio shot by photographer Annie Leibovitz, a film and plush likenesses. Though the zoo has never released exact numbers, Knut merchandise including postcards, key chains, candy and stuffed Knuts have brought in hundreds of thousands of euros.

He was so adored, and profitable, Berlin's zoo paid some euro430,000 ($600,000) to the Neumuenster zoo to settle a financial dispute over his ownership. Neumuenster owns Knut's father and had insisted it was the legal owner of Knut, the elder bear's first offspring.

No longer a cub, Knut grew rapidly, weighing a hulking 440-pounds (200-kilograms) by age two, and trading in white fluff for yellowish fur. Doerflein, the zookeeper who raised him, died in 2008 of a heart attack, earning front page headlines in German newspapaper as "Knut's daddy."

Between 600 and 700 people were at his compound when Knut died, zoo officials said. One visitor said she watched Knut lying on the surface of the water motionless with only his back showing for ten 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.

Some said they feared Knut had died from all the stress that stardom brings.

"I suspect he died of a heart attack," said Elke Neumann, who had come to Knut's enclosure when she heard of his death. "And I hope the medical officials will be able to confirm that it was because of stress."

File photo of Berlin zoo employee Thomas Doerflein playing with polar bear cub Knut during the bear's first presentation in Berlin zoo, March 23, 2007. [Photo/Agencies]