Google computer program AlphaGo has defeated its human opponent, South Korean Go champion Lee Sedol, in the first face-off of a historic five-game match.
AlphaGo's victory in the ancient Chinese board game is a breakthrough for artificial intelligence, showing the program developed by Google DeepMind has mastered one of the most creative and complex games ever devised.
Commentators say the match on Wednesday was close - AlphaGo and Lee made mistakes and the result was unpredictable until near the end.
Lee's loss was a shock to South Koreans and Go fans. The 33-year-old was confident at first of a sweeping victory two weeks ago, but sounded less optimistic a day before the match.
"I was very surprised because I did not think that I would lose the game. A mistake I made at the very beginning lasted until the very last," said Lee, who has won 18 world championships since becoming a professional Go player at age 12.
Lee said AlphaGo's early strategy was excellent and he was stunned by one unconventional move it made that a human never would have.
The loss shook the South Korean Go community. Yoo Chang-hyuk, also a South Korean Go master, said it was a big shock.
"It did not play like a human at all," Kim Sung-ryong, another Go expert, said of the computer's lack of emotion despite making potentially fatal mistakes.
Hundreds of thousands of people watched the game live on TV and YouTube. The remaining four matches will end on Tuesday.
Computers conquered chess in 1997 in a match between IBM's Deep Blue and chess champion Garry Kasparov, leaving Go as "the only game left above chess", DeepMind chief Demis Hassabis said.
Top human players rely heavily on intuition and feelings to choose among a near-infinite number of board positions in Go, making the game extremely challenging for the artificial intelligence community.
AI experts had forecast it would take another decade for computers to beat professional Go players. That changed when AlphaGo defeated a European Go champion in 2015 in a closed-door match published in the journal Nature. AlphaGo's performance has steadily improved since.
