British singer Robbie Williams performs before the group A match between Russia and Saudi Arabia which opens the 2018 soccer World Cup at the Luzhniki stadium in Moscow, Russia, Thursday, June 14, 2018. (AP Photo/Darko Bandic)

London/Moscow — British popstar Robbie Williams appeared to tell critics that he performed at Thursday's opening ceremony of the football World Cup "for free," showing his middle finger to a live television camera.

"I did it for free," Williams sang as he moved close to a camera during "Rock DJ," making his one-fingered gesture in a possible response to accusations that he had accepted Russian government money for singing at Moscow's Luzhniki Stadium.

Williams and Russian opera soprano Aida Garifullina teamed up for a duet in a 15-minute ceremony ahead of the opening match between hosts Russia and Saudi Arabia, singing Williams' hit "Angels" as Russian President Vladimir Putin and other dignitaries watched.

Bill Browder, the founder of Hermitage Capital Management and an outspoken critic of Putin, was among those who had criticized Williams.

"In addition to legitimizing Putin by performing at the World Cup, @robbiewilliams performed for Russian oligarchs Alisher Usmanov and Roman Abramovich in Russia in the past," Browder tweeted on Wednesday.

Williams also performed his hits "Let Me Entertain You" and "Feel" in Thursday's ceremony before the Russian and Saudi Arabian teams entered the pitch for the first of 64 matches.

He said earlier that he was "happy and excited to be going back to Russia for such a unique performance."

"I've done a lot in my career, and opening the FIFA World Cup to 80,000 football fans in the stadium and many millions all over the world is a boyhood dream," Williams said on his official website.