(Adds industry, consumer group, Capitol Hill reaction)

By David Shepardson

WASHINGTON, Jan 23 (Reuters) - President Donald Trump on Monday tapped Republican Commissioner Ajit Pai to head the U.S. Federal Communications Commission, which is expected to roll back many of the Obama Administration's telecommunications and internet policies.

Pai, a former Justice Department, FCC and Capitol Hill staffer, in December predicted landmark "net neutrality" rules adopted in 2015 would not last. The son of Indian immigrants who grew up in Kansas, Pai said the commission should take a "weed whacker" to unneeded rules and was harshly critical of many FCC regulations imposed during the Obama administration.

"During the Trump administration, we will shift from playing defense at the FCC to going on offense," he said in December. "We need to fire up the weed whacker and remove those rules that are holding back investment, innovation, and job creation."

Pai will also have a key role in deciding whether to approve or reject or impose conditions on mergers involving cable and telephone companies. In May, Pai opposed FCC conditions imposed on Charter Communication's acquisition of Time Warner Cable.

Last week, then FCC chairman Tom Wheeler urged Republicans against dismantling internet access protections that bar service providers from slowing consumer access to web content.

Internet providers fear net neutrality rules make it harder to manage internet traffic and make investment in additional capacity less likely. The Republican-controlled Congress is also considering rewriting the net neutrality rules.

Pai said in 2015 that consumers would be worse under net neutrality and should "expect their bills to go up, and they should expect that broadband will be slower going forward."

Senator Maria Cantwell, a Washington state Democrat, criticized Pai in a letter to President Donald Trump Monday as "not a supporter of the FCC's strong rules to protect an open internet, putting at risk our robust net neutrality rules and the three million internet economy jobs it supports."

Under Wheeler, the FCC and major telecom and cable companies have been at odds over a number of big issues in recent years, including tougher broadband privacy rules and a proposal to allow pay-TV consumers to ditch set top boxes.

Comcast Corp, AT&T Inc, Verizon Communications Inc, Frontier Communications Corp, T-Mobile US Inc and Charter all praised Pai's appointment in separate statements.

"Chairman Pai will work with his fellow commissioners to quickly and decisively put back in place the commonsense regulatory framework necessary to support the president's agenda," AT&T said.

However, advocacy group Free Press President Craig Aaron said Pai "has been on the wrong side of just about every major issue that has come before the FCC during his tenure. He's never met a mega-merger he didn't like or a public safeguard he didn't try to undermine."

U.S. Telecom, an industry trade group, said in a statement it shares Pai's vision for a "future based on a bold but pragmatic strategy to erase the many regulatory barriers impeding the expansion of our nation's communications infrastructure, and the jobs and economic opportunity that depend on it."

Pai does not need Senate confirmation as chairman but his current term expires at year end and he will need to be reconfirmed to continue.

(Reporting by David Shepardson; Editing by Meredith Mazzilli, Bernard Orr)