Google and Facebook took particularly sharp jabs for alleged abuse of their market power from Democrats and Republicans in a much-anticipated congressional hearing that put four of the most prominent tech CEOs in the United States in the hot seat.

The chairman of the US House of Representatives antitrust panel holding the hearing on Wednesday said afterwards the four CEOs had acknowledged behaviour which had caused concern.

"What we heard from witnesses at the hearing confirmed the evidence that we have collected over the last year," Representative David Cicilline, a Democrat, told the Reuters news agency.

Facebook Inc's Mark Zuckerberg, Amazon.com Inc's Jeff Bezos, Google owner Alphabet Inc's Sundar Pichai and Apple Inc's Tim Cook - whose companies have a combined market value of about $5 trillion - parried a range of accusations that they crippled smaller rivals in their quest for market share.

The hearing via video conference was the first time the four CEOs have appeared together before legislators.

Though it was Bezos's first congressional testimony, he appeared the least fazed. Cook drew fewer barbed questions than Bezos and handled them well. Zuckerberg suffered the most damage, stumbling at times when confronted with internal emails.

Pichai, CEO of both Alphabet and Google, took the most heat from conservatives on the panel and looked the worse for it, as he repeatedly told lawmakers he would be happy to look into various situations and get back to them.

The Big Tech hearing triggered scorn from viewers over its own tech issues. Bezos escaped questioning for about an hour in what may have been a tech issue and was caught on screen reaching for what appeared to be a snack.

Poor audio, flat-screen televisions switching off, and chief executives appearing together as thumbnails on a large screen led to mockery of the virtual set-up on Twitter.

Legislators descended into shouting at each other at points, with a pandemic twist. One yelled: "Put your mask on!"