In his first formal congressional appearance, the Facebook founder and chief executive sought to quell the storm over privacy and security lapses at the social media giant that have angered lawmakers and the network's two billion users.
Under mounting pressure over the hijacking of its user data by a British political consultant, Zuckerberg reiterated his apology for the historic breach, before being grilled over how Facebook collects and protects people's personal information.
"It was my mistake, and I'm sorry," Zuckerberg said. "I started Facebook, I run it, and I'm responsible for what happens here."
"It's clear now that we didn't do enough to prevent these tools from being used for harm," he said. "That goes for fake news, foreign interference in elections, and hate speech, as well as developers and data privacy."
The 33-year-old CEO spoke of a constant struggle to guard against Russian manipulation of the Facebook platform to influence elections in the US and elsewhere.
"There are people in Russia whose job it is to try to exploit our systems and other internet systems and other systems as well," he said.
"So this is an arms race. They're going to keep getting better and we need to invest in getting better at this too."
Zuckerberg has previously acknowledged the social network failed to do enough to prevent the spread of disinformation during the last US presidential race.
"After the 2016 election, our top priority was protecting the integrity of other elections around the world," he said.
Work With Special Counsel
Zuckerberg also revealed that Facebook is cooperating with the US special prosecutor investigating Russian interference in the 2016 vote.
"Our work with the special counsel is confidential. I want to make sure in an open session I don't reveal something that's confidential," he said.
"I believe there may be (a subpoena), but I know we're working with them," he said.
Swapping his customary T-shirt for a business suit and tie, Zuckerberg faced tough questions over how a US-British political research firm, Cambridge Analytica, plundered detailed personal data on 87 million users to be used in the 2016 US presidential election.
Reiterating comments from the past few days, he took responsibility for the data misuse.
'Doesn't Feel' Like A Monopoly
Dozens of protestors gathered outside Congress before the hearing wearing Zuckerberg masks and #DeleteFacebook T-shirts.
Inside the jammed hearing room, activists from the Code Pink group wore oversized glasses with the words "STOP SPYING" written on the lenses, and waved signs that read "Stop corporate lying."
Testifying was a new step forward for Zuckerberg, who started Facebook as a Harvard dropout in 2004, and built it into the world's largest social media company worth $470 billion.
During questioning, Zuckerberg rejected the suggestion that the social media giant, with over two billion users worldwide, has exclusive control over its market.
"It certainly doesn't feel like that to me," he said when asked if he thinks Facebook has a monopoly.
"The average American uses eight different apps to communicate with their friends and stay in touch with people. Ranging from texting apps to e-mail," he said.
But when asked if Facebook favored regulation of online firms, he said, "I think if it's the right regulation, yes."
Lawmakers have suggested the election meddling and poor controls on personal data requires the government to step in to regulate Facebook and other social media companies which generate revenue from user data.
"The status quo no longer works," said Senator Chuck Grassley, chair of one of the committees holding the hearing.
"Congress must determine if and how we need to strengthen privacy standards to ensure transparency and understanding for the billions of consumers who utilize these products."
"You have a real opportunity this afternoon to lead the industry and demonstrate a meaningful commitment to protecting individual privacy," Democratic Senator Dianne Feinstein told Zuckerberg at the rare joint committee hearing, to be followed by a similar hearing in the House of Representatives on Wednesday.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)