Critics accused Britney Spears of lip-syncing during the opening night of Las Vegas residency show Britney: Piece of Me on Dec. 27.

The pop star was performing at the Planet Hollywood Resort and Casino when it appeared that she was not using her real voice.

Spears's manager, Larry Rudolph, previously said lip-syncing would not be happening during the two-year show. He insisted, "There might be some numbers where she's full out dancing with a [vocal] track underneath her, but there won't be any lip-syncing across the board."

However, critics who came to review Spears's concert had strong assumptions about the singer not singing live the whole show. Caryn Ganz from Rolling Stone wrote, "While the show's producers claim Spears is singing along with backing tracks of her own voice, there was little evidence Britney sang a live note."

Mikael Wood from The Los Angeles Times felt the same way: "The singer's presence felt so diminished - her dancing a tentative shadow of what it used to be, her vocals apparently lip-synced for the majority of the show."

Keith Caulfield from Billboard.com also made a comment about Spears not singing live. He wrote, "The vast majority of the show was lip-synced, and only occasionally were any seemingly live vocals heard. There was a certain thrill in actually hearing Spears breathing into the microphone -- however infrequent."

Marco della Cava from USA Today made a slightly different statement, noting the tracks make it difficult to really know when a voice is real and when it's not because of their different effects.

"As for the oft-raised specter of lip-syncing, it's impossible to gauge whether there was any funny business with the vocals," he wrote. "These club-oriented tunes all require such different effects that her voice surely had to be run through computers to correspond with the tone of what was recorded in the studio."

Britney's Opening Night in Las Vegas, Performance VIDEOS [Watch]

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Britney Spears, Concert, Music, Las Vegas