(Adds opposition group)

By Alina Selyukh

WASHINGTON, Dec 3 (Reuters) - U.S. Federal Communications Commission on Wednesday restarted its informal 180-day countdown to review the proposed mergers between Comcast Corp and Time Warner Cable Inc, and AT&T Inc and DirecTV.

The FCC's self-imposed, non-binding "shot clock" will restart at Day 70 for the AT&T-DirecTV merger and Day 85 for the Comcast-Time Warner Cable merger, the agency said.

The agency had paused the clock on the reviews as it figured out how to properly consider and allow the public to review sensitive documents related to agreements with media companies.

The FCC will now restart its collection of public comments on various aspects of the merger, allowing some reviewers to access highly confidential information that is not related to video programming agreements.

The U.S. Court of Appeals for the District of Columbia Circuit, at the request of numerous content companies, is weighing how the FCC should handle the confidential materials related to programming agreements.

Once the court rules, the FCC indicated it may further reset or delay the deadline to ensure enough time for outside parties to comment and for the agency to properly consider those comments.

The country's biggest cable company Comcast has bid $45 billion for the No. 2 Time Warner Cable, while second-largest wireless provider AT&T has bid $48.5 billion to buy biggest satellite TV provider DirecTV.

The FCC has to determine whether the deals are in the public interest. It is reviewing the mergers alongside the Justice Department, whose antitrust review also includes sensitive programming documents but is confidential.

A group of critics of the Comcast-Time Warner Cable merger on Wednesday announced a coalition to press regulators to block the transaction. The "Stop Mega-Comcast" coalition includes satellite company Dish Network Corp, consumer advocacy group Public Knowledge and TheBlaze, conservative commentator Glenn Beck's media company.

Critics say the merger would create a cable and Internet provider that would have too much control over what Americans watch on TV and their web activity, among other things.

Comcast has stressed that it does not compete against Time Warner Cable in any market and that, together, they would offer better services to more consumers.

(Reporting by Alina Selyukh; Editing by Chizu Nomiyama and Gunna Dickson)