UPDATE 4-Facebook CEO Zuckerberg starts testifying in U.S. Senate hearing

(Updates with start of hearing)

WASHINGTON/SAN FRANCISCO, April 10 (Reuters) - Facebook Inc Chief Executive Mark Zuckerberg started testifying before Congress on Tuesday as he looks to fend off the possibility of new regulations as a result of the privacy scandal engulfing his social network.

The 33-year-old internet mogul faces tough questioning from a joint hearing of the U.S. Senate's Commerce and Judiciary committees.

John Thune, chairman of the U.S. Senate Commerce, Science and Transportation committee, struck an adversarial tone in his opening remarks https://www.commerce.senate.gov/public/index.cfm/hearings?Id=E7C85E5C-BBA9-4AD9- 8 6 1 D - 8 5 B 7 3 4 0 B B D F B & S t a t e m e n t t i d = F F 2 F A D D A - E 1 B 9 - 4 6 B E - 8 F B 7 - 0 C E 9 8 D D A 1 8 F A .

"In the past, many of my colleagues on both sides of the aisle have been willing to defer to tech companies efforts to regulate themselves. But this may be changing," he said.

Hours before the hearing, people waited in a line inside the Hart Senate Office Building, set off by velvet ropes, stretching from the briefing room down a corridor. Some brought folding chairs, while others stood or sat on the floor.

Outside the Capitol building, which houses Congress, online protest group Avaaz set up 100 life-sized cutouts of Zuckerberg wearing T-shirts with the words 'Fix Facebook.'

Zuckerberg, who founded Facebook in his Harvard University dorm room in 2004, is fighting to prove to critics that he is the right person to go on leading what has grown into one of the world's largest companies.

Facebook faces a growing crisis of confidence among users, advertisers, employees and investors after acknowledging that up to 87 million people, mostly in the United States, had their personal information harvested from the site by Cambridge Analytica, a political consultancy that has counted U.S. President Donald Trump's election campaign among its clients.

On Friday, Zuckerberg threw his support behind proposed legislation requiring social media sites to disclose the identities of buyers of online political campaign ads.

Facebook shares were up 2.5 percent in afternoon trading. (Reporting by David Ingram in San Francisco and Dustin Volz in Washington Additional reporting by David Shepardson in Washington and Andy Sullivan in Washington Editing by Bill Rigby and Meredith Mazzilli)