A college student has been shot dead while playing Pokemon Go at a San Francisco tourist attraction.

The authorities say 20-year-old Calvin Riley was shot on Saturday night by an unknown assailant at Aquatic Park near Ghiradelli Square.

The US Park Police and local murder detectives are investigating what led to the shooting.

A family friend told KGO-TV that Mr Riley and a friend were playing the popular mobile game when someone came up and shot the young man in the back and ran away.

“From what we know there was no confrontation. There was nothing said back and forth. It was just senseless, just came up and shot in the back and ran away for nothing,” John Kirby said.

He said Mr Riley and his friend had noticed someone watching them from the top of a hill that overlooks the park, but it was dark and they were mostly looking at their mobile phones.

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National Park Service spokesman Lynn Cullivan said investigators believe there are witnesses to the attack because it happened in the bustling Fisherman's Wharf area.

Sgt Robert Jansing, a detective with the US Park Police, said the gunman did not take any of Mr Riley's belongings or try to rob him.

Pokemon Go has become an international sensation this summer as legions of fans visit real-world landmarks in order to find and trap cartoon creatures.

But the game's popularity has created unintended consequences in everyday life. Property owners complain of players trespassing or damaging their grounds and scores of players have been injured or robbed as they stare at their phones in search of digital monsters.