The YouTube live stream of a US congressional hearing on white supremacism was forced to disable the comments section after it was overwhelmed by users spouting racist remarks against Jews and other minority groups.
Executives from Google, which owns YouTube, and Facebook were among those testifying before the House Judiciary Committee on Tuesday.
Both companies said they were employing a combination of artificial intelligence and human employees to combat hate speech on their platforms.
"Hate speech and violent extremism have no place on YouTube," said Alexandria Walden, a counsel on human rights at Google.
She said the company was working to swiftly remove content that violates hate speech, while diminishing the prominence of borderline cases.
The shooter in the New Zealand mosque massacre used Facebook's live stream to broadcast his crimes, drawing more criticism of the company's handling of hate.
Source: Press Association
One of the witnesses testifying was Mohammad Abu-Salha, a doctor whose children were killed by a suspected anti-Muslim bigot in 2015.
Jerry Nadler, the chairman of the committee and a leading Democrat, blamed President Donald Trump for an attitude that "fans the flames with language that, whether intentional or not, may motivate and embolden white supremacist movements".
The Anti-Defamation League, a leading Jewish group focused on hate crimes, said 78 per cent of violent extremist criminal acts in the US last year were by white supremacists.
"We are seeing a resurgence, where most of the crimes are from white extremists," said Eileen Hershenov from the ADL, warning that social media was being used to spread hate.
The Republicans invited controversial figures to testify, including Candace Owens, a black activist who recently appeared to justify some of Adolf Hitler's domestic policies, in remarks she later walked back.
