Thousands of anti-Brexit protesters gathered outside Britain's parliament cheered as lawmakers voted on Saturday to withhold support for Prime Minister Boris Johnson's EU exit deal until formal ratification legislation had passed.

"Hear us," many of the protesters said outside the 800-year-old parliament building while others chanted "People's vote."

Hundreds of thousands of Britons had marched through central London towards parliament to demand a new Brexit referendum, with many crowding into the grass square outside to watch the result on a big screen.

"It's something that needs a lot of scrutiny and it's a mini win for the remainers. Everything is so finely balanced," said John Warman, 53, a marine adjuster from West Byfleet.

Bill Venables, 58, who works for the local authority in Bristol in western England, said: "It's good news that the opposition to Boris Johnson's deal is solid because there were signs that it would melt away but it hasn't."