Facebook CEO Mark Zuckerberg departs after meetings with senators on Capitol Hill, Monday, April 9, 2018, in Washington. Zuckerberg will testify Tuesday before a joint hearing of the Commerce and Judiciary Committees about the use of Facebook data to target American voters in the 2016 election. (AP Photo/Alex Brandon)

Capitol Hill - Facebook chief Mark Zuckerberg was set for a fiery face-off on Capitol Hill Tuesday as he attempts to quell a storm over privacy and security lapses at the social network that have angered lawmakers and its two billion users.

Zuckerberg, making his first formal appearance at a Congressional hearing, will seek to allay widespread fears ignited by the leaking of private data on tens of millions of users to a British firm working on Donald Trump's 2016 campaign.

The scandal has sparked fresh calls for regulation of social media platforms, and Facebook in the past week has sought to stem criticism by endorsing at least one legislative proposal, which would require better labeling and disclosure on political advertising.

Senator Bill Nelson, one of several lawmakers who met privately Monday with Zuckerberg, said he believes the 33-year-old CEO is taking the matter seriously.

"I believe he understands that regulation could be right around the corner," the Florida Democrat said.

Other lawmakers were less clear about the need for new regulations.

Republican Senator John Kennedy of Louisiana said, "I'm not interested in regulating Facebook. I'm interested in Facebook regulating itself and solving the problems. I come in peace."

Zuckerberg was to appear before a Senate panel from 1815 GMT, with another session in the House of Representatives Wednesday.

The huge social network has begun alerting some users about whether their data was leaked to the British firm Cambridge Analytica.

Notification is among several steps pledged by Facebook to fix pervasive problems on data security and manipulation of the giant platform.

Senate Judiciary Chairman Charles Grassley said Tuesday's hearing is the first step in an "open dialogue about how we address growing consumer privacy concerns."  

"The tech industry has a duty to respond to widespread and growing privacy concerns and restore the public trust. The status quo no longer works," Grassley added.

Facebook meanwhile unveiled a "bounty" program offering rewards to people who report misuse of private information on the platform, offering at least $500 for verified cases affecting at least 10,000 users.

The program "will reward people with first-hand knowledge and proof of cases where a Facebook platform app collects and transfers people's data to another party to be sold, stolen or used for scams or political influence," product security chief Collin Greene said in a statement.

- Protests, cutouts -

Ahead of Tuesday's hearing, the activist group Avaaz set up life-sized cutouts of Zuckerberg wearing a "Fix Facebook" T-shirt on the Capitol lawn, urging him to do more to root out fake accounts and "bots."

Outside the Senate hearing room, scores of people lined up including some anti-Facebook protesters.

"I hope that Mr Zuckerberg is just as honest as possible, way more honest than he has been in the past with his statements," said Annamarie Rienzi, 22, wearing a #DeleteFacebook T-shirt.

On Monday, Zuckerberg ditched his trademark T-shirt for a somber dark suit and tie as he made the rounds on Capitol Hill and sounded contrite about Facebook's conduct.

"We didn't take a broad enough view of our responsibility, and that was a big mistake. It was my mistake, and I'm sorry," Zuckerberg said in written testimony released by the House commerce committee. 

In his remarks, Zuckerberg called Facebook "an idealistic and optimistic company" and said: "We focused on all the good that connecting people can bring."

But he acknowledged that "it's clear now that we didn't do enough to prevent these tools from being used for harm as well. That goes for fake news, foreign interference in elections, and hate speech, as well as developers and data privacy."

Zuckerberg added: "I want to be clear about what our priority is: protecting our community is more important than maximizing our profit."

- 'Investigating every app' -

The Facebook CEO recounted a list of steps aimed at averting improper use of data by third parties like Cambridge Analytica, and noted that other applications were also being investigated to determine if they did anything wrong.

On Friday, Facebook sought to allay concerns over political manipulation of its platform by announcing support for the "Honest Ads Act" that requires election ad buyers to be identified, and to go further by verifying who sponsors ads on key public policy issues.

Zuckerberg vowed to "hire thousands of more people" to get the new system in place ahead of US midterm elections in November, starting the process in the United States and taking it global in the coming months.