Superstar Madonna headlined the mid-game entertainment for professional football's championship, which is seen by more than 100 million U.S. television viewers, but she received only mixed reviews. Indeed, the day-after reckoning focused on rapper M.I.A.'s rude act and drew instant comparisons to Janet Jackson's infamous "wardrobe malfunction" in 2004.

At Sunday's show, M.I.A. joined Madonna on stage with U.S. hip-hop star Nicki Minaj and was performing the cheerleader themed "Give Me All Your Luvin'" from Madonna's latest album when M.I.A. extended her middle finger in a fleeting, obscene gesture while facing the camera.

NFL spokesman Brian McCarthy said in a statement that NBC was to blame for a technical failure.

"There was a failure in NBC's delay system. The gesture in the performance was completely inappropriate, very disappointing, and we apologize to our fans," he said.

NBC sports spokesman Christopher McCloskey shifted more of the blame back to the NFL, saying, "The NFL hired the talent and produced the halftime show. Our system was late to obscure the inappropriate gesture and we apologize to our viewers."

Madonna was the first female Super Bowl halftime headliner since Jackson's performance of 2004, during which fellow performer Justin Timberlake tugged at her costume, exposing her nipple to millions of TV viewers and causing an uproar. Afterward, Jackson characterized it as a "wardrobe malfunction," but parents groups and others were in an uproar.

Sunday's Superbowl broadcast of the New York Giants 21-17 victory over the New England Patriots drew a slightly lower audience than last year's record-breaking 111 million viewers, according to preliminary figures.

Full data will be released later on Monday, but according to overnight Nielsen ratings, the game drew a 47.8 household rating, slightly down on 2011's 47.9 household rating.

As billed, the game proved to be a close one, but nobody in the audience expected the gesture by M.I.A. In fact, in a pre-Super Bowl press conference, Madonna promised there would be no slip-ups on the order of Jackson's costume snafu.

Activist group, the Parents Television Council said on Monday both the NFL and NBC were to blame for M.I.A.'s actions and asked for further immediate steps to be taken "to hold those accountable for this offensive material."

"The network cannot say it was caught off guard. It has been eight years since the Janet Jackson striptease, and both NBC and the NFL knew full well what might happen," the PTC said.

"They chose a lineup full of performers who have based their careers on shock, profanity and titillation. Instead of preventing indecent material, they enabled it," the group added.

Outside M.I.A.'s action, Madonna's glitzy, Cleopatra-themed performance divided critics and viewers. Singer Sean "P.Diddy" Combs tweeted that Madonna "had the best Half-time performance of all time !!".

But the Baltimore Sun's David Zurawik called it a "joke of a halftime show featuring an embalmed version of Madonna snatched off the undertaker's table."

The halftime show has increasingly featured high-profile pop acts, a far cry from the first Super Bowl in 1967 when college marching bands entertained the crowd.

Paul McCartney, U2, Prince, The Rolling Stones, Bruce Springsteen and The Black Eyed Peas have been among the other recent performers.

(Reporting By Christine Kearney, additional reporting by Chris Michaud; Editing by Jill Serjeant and Bob Tourtellotte)

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