(Recasts with updated figures)

NEW YORK, Sept 9 (Reuters) - Stephen Colbert drew an audience of 6.6 million people in his long-awaited debut as host of "The Late Show" on CBS but he largely failed to bowl over TV critics with any major changes to the late-night talk show format.

"The Late Show" was, as expected, the most-watched late-night TV program on Tuesday, attracting more than double the 2.9 million audience for rival Jimmy Fallon's "Tonight" show on Comcast Corp's NBC, Nielsen data showed on Wednesday.

Colbert's 6.6 million audience was small in comparison to Fallon's February 2014 debut as host of the "Tonight" show, which was watched by some 11.3 million Americans.

But Colbert saw a boost of up to 200 percent in the numbers of viewers under age 34, compared with last year's season premiere of "The Late Show" when David Letterman was behind the desk, ratings data showed.

Colbert had been off the air since December when "The Colbert Report" ended on cable channel Comedy Central. Although his return won generally favorable reviews, some critics were disappointed.

James Poniewozik at the New York Times called the first show "overstuffed and messy."

But he added; "This show may not completely know what it is yet, but it knows exactly who its host is: a smart, curious, playful entertainer who's delighted to be there."

Robert Bianco said in USA Today Colbert "seemed a bit over-caffeinated. But calm will almost certainly come with time."

Variety's Brian Lowry said "if the goal was to establish the CBS show as fun-loving (a silly bit with George Clooney) yet potentially topical (an interview with Jeb Bush), as another Bush family member might say, 'Mission accomplished.'"

The Chicago Tribune was unimpressed, calling Colbert's debut "inauspicious."

"Tuesday night's debut, so highly anticipated, so long in the making, came off as yet another frantic yet fundamentally formulaic iteration of your grandparents' late-night talk show. There was very little that was sly and almost nothing that was subversive about the effort," the Tribune's Eric Zorn wrote.

At the Washington Post however, Amber Phillips said two things were clear from Colbert's debut:

"1) Colbert plans to be a major player at the nexus of pop culture and the 2016 presidential election, and 2) he's going to take politics and its players seriously." (Reporting by Jill Serjeant; Editing by Eric Walsh and Mohammad Zargham)

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