(Adds detail on protest)
SAN FRANCISCO/WASHINGTON, April 10 (Reuters) - Facebook Inc Chief Executive Mark Zuckerberg will strike a conciliatory tone on Tuesday in testimony before Congress 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 is set to appear in Washington before a joint hearing of the U.S. Senate's Commerce and Judiciary committees some 15 or 20 minutes after the originally scheduled time of 2:15 p.m. (1815 GMT) because of a Senate vote.
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' on them.
Zuckerberg, who founded Facebook in his Harvard University dorm room in 2004, is fighting to demonstrate 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.
Zuckerberg, who has never testified in a congressional hearing, said in written testimony on Monday that he had made mistakes and had held too narrow a view of the social network's role in society.
"Now we have to go through every part of our relationship with people and make sure we're taking a broad enough view of our responsibility," he said.
Facebook hired several outside consultants to help coach Zuckerberg, even holding mock sessions to prepare him for questions from lawmakers.
In an olive branch on Friday, Zuckerberg threw his support behind proposed legislation requiring social media sites to disclose the identities of buyers of online political campaign ads. Twitter Inc also said on Tuesday, for the first time, that it supports the bill, called the Honest Ads Act.
U.S. lawmakers have discussed legislation that would strengthen data privacy protections and enforcement. Tighter regulation of how Facebook uses its members' data could affect its ability to attract advertising revenue, its lifeblood.
Some 40 senators out of the 100-member Senate sit on the two committees holding Tuesday's hearing, setting up a possibly marathon hearing.
To ease the way, Zuckerberg on Monday met some lawmakers privately, listening to their concerns before they will have a chance to interrogate him in public.
Zuckerberg appeared willing "to turn things around where he sees mistakes that have been made," Senator Bill Nelson, the top Democrat on the Senate Commerce Committee, said after meeting with the CEO.
For hearings last year about Russia's alleged use of social media to influence American politics, Facebook, Twitter and Alphabet Inc's Google sent lawyers, angering lawmakers.
Zuckerberg may face a torrid time from some senators. On Tuesday, Democratic Senator Chris Coons, a member of the Judiciary committee, complained via Twitter about fake profiles.
"On today of all days, I just found out that there are two fake Facebook accounts impersonating me, and guess what? Many of the friends appear to be Russian accounts," he tweeted. "zfacebook and Mr. Zuckerberg-this is unacceptable."
Zuckerberg will get a second dose of questioning on Wednesday from the U.S. House Energy and Commerce Committee.
His perch atop Facebook is assured as long as he wants it, given that he remains its controlling shareholder.
But his reputation has suffered as television comedians have mocked his perceived robotic speaking patterns and allegedly cavalier attitude toward privacy.
Shares in Facebook are down more than 14 percent since the Cambridge Analytica scandal broke last month. (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 Cynthia Osterman and Bill Rigby)