CAPE TOWN, South Africa –  Corrie Sanders, the South African southpaw who knocked out Wladimir Klitschko in one of the great upsets in heavyweight boxing, has died after being shot by robbers at a restaurant during a family celebration.

The former WBO and WBU champion was 46.

Sanders was shot in the hand and stomach at a family member's 21st birthday gathering at the restaurant in Brits, in South Africa's North West province, on Saturday night, police said. He died in a hospital in the capital city Pretoria in the early hours of Sunday morning.

"It's very sad," Sanders' former trainer Harold Volbrecht told a South African radio station. "My heart is broken. It's just terrible. I can't believe it happened."

Police Brigadier Thulani Ngubane said three armed men entered the restaurant with the apparent intention to rob it and shot randomly, hitting Sanders. No other injuries were reported. The robbers took a cellphone and a bag from customers, Ngubane said.

No arrests had been made but a murder investigation has been opened.

Renowned for his impressive hand speed, the 6-foot-4 (1.93-meter) Sanders was one of South Africa's most successful and popular fighters after a near 20-year professional career that included the South African, WBU and WBO heavyweight title belts.

He retired in 2008 with a 42-4 record, with 31 knockouts, but was remembered for the shock second-round knockout of Klitschko in Germany in 2003 that earned him the WBO title and respect across the world as a wily, fast and clever fighter as well as a powerful puncher.

He then lost to Wladimir's older brother Vitali for the vacant WBC title the following year, Vitali's first fight for the title he still holds. Sanders had earlier relinquished his WBU crown in a seventh-round stoppage by Hasim Rahman after being ahead in their fight in 2000.

South African sports figures, television personalities and entertainers all paid tribute on Sunday to the popular Sanders, who also was a single handicap golfer.

"Shocking news about Corrie Sanders' death, gonna miss u Corras," South Africa limited-overs cricket captain A.B. de Villiers posted on Twitter from the World Twenty20 tournament in Sri Lanka.

South African rugby player Morne Steyn wrote: "All the best for Corrie Sanders family and friends! We lost one of SA's legends! God Bless!"

Olympic swimming champion Cameron van der Burgh added: "RIP Corrie. A champion of the World."

Born in Pretoria, Sanders began his professional boxing career in 1989 with a first-round TKO of King Kong Dyubele. Eighteen of the fast-starting Sanders' 31 career knockouts came in the first round.

He claimed the South African heavyweight title in 1991, but his demolition of Wladimir Klitschko in Hanover in 2003 -- when he knocked the champion down twice in the first round -- was by far his best and most famous fight.

It is one of only three losses for the Ukrainian and current WBO champion. Sanders only agreed to the fight at short notice, was a 40-1 underdog and the result was voted as the upset of the year by Ring Magazine.

Sanders hung up his gloves in 2008 after being knocked out by Osborne Machimana for the South African heavyweight title -- ironically in the first round.