Britney Spears' first major performance in three years, at the MTV Video Music Awards in Las Vegas, has been widely dismissed as a disaster.
The BBC's David Willis said her performance would "go down in the history books as being one of the worst to grace the MTV Awards".
Her ex-boyfriend Justin Timberlake and R&B star Rihanna were the big winners.
Timberlake took four awards, including top male artist, while Rihanna scooped video of the year for Umbrella.
Spears, who was not nominated for any awards, opened the show dressed in a sequinned bikini and knee-high boots, performing her new single Gimme More.
Our correspondent said the performance would hit the headlines "for all the wrong reasons".
The singer looked "really out of sorts" and "looked as though she completely lost the art of lip-synching", he said.
Celebrity gossip blogger Perez Hilton said: "She blew it. Everybody knows Britney lip-synchs, but that's because she dances so much.
"She barely even danced in this. It was so bad. It was painful. It was embarrassing. And I loved it!"
Singer and producer Akon added that "it definitely could have been a lot better".
MAIN MTV AWARD WINNERS
Video of the year - Rihanna, Umbrella
Male artist of the year - Justin Timberlake
Female artist of the year - Fergie
Best group - Fall Out Boy
"She seemed nervous... you could tell by the expression on her face," he said.
"Instead of just blocking everybody out and doing her thing, you could tell she was thinking about it."
The performance comes after a year when Spears publicly shaved her head, went into rehab and fought husband Kevin Federline for custody of their two sons.
Rihanna's Umbrella, which spent 10 weeks at number one in the UK, took MTV's single of the year award, beating Beyonce, Timberlake, Kanye West and Amy Winehouse.
"Wow - video of the year," said Rihanna. "This was definitely the most important one for me of the night... the most unexpected for me."
Timberlake was the night's biggest winner, collecting four prizes
Timberlake, who was named best male artist, said: "It's just exciting to see a new generation of artists really doing it, and doing it well.
"I think music is in a great place right now because we're all doing what we want to do. So thank you for this award, MTV."
Fergie was named best female artist, while Beyonce and Shakira took the award for most earth-shattering collaboration for Beautiful Liar.
The best new artist gong went to Gym Class Heroes, and Fall Out Boy were named best group.
Rapper Kanye West threw a tantrum in front of media after missing out on the five awards he was nominated for.
"That's two years in a row, man... give a black man a chance," West said. "I'm trying hard, man, I have the... number one record, man."
Rihanna also performed during the ceremony in Las Vegas
West, who gatecrashed the stage at the MTV Europe Awards after failing to win the prize for best video, said he would never return to MTV.
And there was more drama off-stage when Kid Rock and Motley Crue drummer Tommy Lee tussled in the audience.
Both are former husbands of actress and model Pamela Anderson, who was a presenter at the show.
Security officials allowed rapper Kid Rock to stay for the rest of the ceremony while Lee was dragged out.
The incident inspired plenty of mock rebukes from those in the crowd.
"They say it's only rappers," said MTV VJ Sway. "I told you, rockers fight too."