The chief executives of Facebook, Google and Twitter faced criticism from U.S. lawmakers on Thursday about their approaches to extremism and misinformation in their first appearances before Congress since pro-Trump rioters assaulted the U.S. Capitol on Jan. 6.

"We fled as a mob desecrated the Capitol, the House floor, and our democratic process," said Democratic Representative Mike Doyle. "That attack, and the movement that motivated it, started and was nourished on your platforms," he added.

Facebook Inc Chief Executive Mark Zuckerberg said there was content related to the riot on its platforms but, when asked if the company bore responsibility for the event, he said the company's responsibility was to "build effective systems."

"We did our part to secure the integrity of the election, and then on Jan. 6, President Trump gave a speech rejecting the results and calling on people to fight," he said. He argued that polarization in the country was due to the political and media environment.

Former President Donald Trump was banned by Twitter over inciting violence around Jan. 6, while Facebook has asked its independent oversight board to rule on whether to bar him permanently. He is still suspended from YouTube.

The hearing was virtual but advocacy group SumOfUs erected cut-outs of Zuckerberg, Sundar Pichai, chief executive of Google parent Alphabet Inc; and Twitter Inc CEO Jack Dorsey dressed as Jan. 6 rioters on the National Mall near the Capitol. One showed Zuckerberg as the "QAnon Shaman," a shirtless rioter wearing horns.

Lawmakers speaking at the joint hearing, held by two subcommittees of the House Energy and Commerce Committee, also slammed the companies over the proliferation of COVID-19 and vaccine misinformation and raised concerns about the impact of social media on children's mental health.

"Your business model itself has become the problem and the time for self-regulation is over. It's time we legislate to hold you accountable," said Democratic Representative Frank Pallone, chair of the Energy and Commerce committee.

Republicans on the panel also criticized the tech giants for what they see as efforts to stifle conservative voices.

Some lawmakers are calling for Section 230 of the Communications Decency Act, which shields online platforms from liability over user content, to be scrapped or rejigged. There are several pieces of legislation from Democrats to reform Section 230 that are doing the rounds in Congress, though progress has been slow. Several Republican lawmakers have also been pushing separately to scrap the law entirely.

In written testimony released on Wednesday, Facebook argued that Section 230 should be redone to allow companies immunity from liability for what users put on their platforms only if they follow best practices for removing damaging material.

Pichai and Dorsey said in the hearing they were open to some of the changes in Facebook's proposal. Pichai said there were some "good proposals." Dorsey endorsed some of the suggestions from Zuckerberg but said it would be difficult to distinguish between small and large services.