Putin, 59, known for his stunts, jumped into the ring ostensibly to congratulate Russian Fedor Emelianenko's MMA win over U.S. fighter Jeff Monson in a three-round fight, The Moscow Times reported Monday.
"Dear friends, today is a great holiday for fans of martial arts," said Putin, a judo black belt.
That's when the boos and catcalls began from the crowd of 20,000 at Moscow's Olympiisky Stadium. They didn't let up until the prime minister, visibly shaken, praised Emelianenko as "a real Russian hero," RIA Novosti said.
The incident has gone viral garnering almost 700,000 hits on YouTube, the Times said.
Although a pro-Kremlin blogger, Elena Savva, contended the booing and whistling (a supreme insult) were intended for the bloody, defeated Monson, Alexei Navalny, a crusading anti-corruption Russian blogger, referred to the derisive display as being a comment on Putin and "the end of an era," The New York Times reported.
Putin's perceived impenetrable castle keep will be tested in two weeks when his ruling United Russia party heads into parliamentary elections, the U.S. newspaper said.