US Republican leaders moved to shut down a dramatic, 12-hour sit-in by Democrats who were demanding that Congress vote on gun safety legislation following the Orlando nightclub massacre.

In one of the most extraordinary scenes on the US House floor, Democrats took over the chamber's proceedings for most of the day on Wednesday, prompting Republican Speaker Paul Ryan to gavel the House into session late at night, nearly drowned out by chants from politicians protesting about his effort to bring the sit-in to a close.

The stunning and unruly scene was broadcast live to the world from Democrats' mobile phones, feeds picked up by C-SPAN after Republicans shut down the network's cameras.

Ryan, who dismissed the protest as a "publicity stunt", refused to allow votes on two bills demanded by Democrats: one expanding background checks to include sales at gun shows and on the internet, and another that prevents people on the government's no-fly list or FBI watchlists from buying a gun.

Ryan instead called for votes on unrelated issues as he sought a return to regular order. Democrats shouted "No bill, no break!" referring to efforts to prevent Congress from adjourning for the Fourth of July holiday.

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After the unrelated votes, the presiding officer declared the House in recess and left the chamber, but dozens of Democrats remained.

"There is a stain on the soul of America, and we must heal it, and it begins tonight," freshman Democrat Brendan Boyle told AFP news agency.

Hundreds of people gathered in front of the Capitol Hall building in Washington DC in support of the Democratic 'sit-in' to demand a vote on gun restrictions [Jim Lo Scalzo/EPA] 

The House drama began before noon.

"We have to occupy the floor of the House until there is action," said House Democrat John Lewis - a civil rights icon who marched with Martin Luther King Jr in the 1960s - just before he and dozens of colleagues sat down on the carpeted floor in the well of the chamber.

US politicians, mainly Democrats, have introduced several bills in recent years aimed at reducing gun violence, including legislation to expand background checks - a provision that has broad public support. But none has passed Congress.

"Who has to be shot, and how many have to die before we do anything?" asked Congresswoman Robin Kelly of Illinois.

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The chamber was scheduled to wrap up business on Thursday before going on break.

The sit-in, which quickly grew to about 100 members, drew the attention of the White House.

"Thank you John Lewis for leading on gun violence where we need it most," Obama posted on Twitter.

"House Republicans may have cut the cameras, but they can't cut off our voices," Democratic White House hopeful Hillary Clinton added in her own tweet. "We have to act on gun violence."

Democratic senators joining the protest included Minority Leader Harry Reid, Bernie Sanders and Chris Murphy, who had waged a nearly 15-hour filibuster last week to force votes in the Senate on gun legislation. Those votes failed on Monday night.

Senators have since moved on to a potential compromise that Republican Senator Susan Collins drafted with a group of bipartisan senators, that would prevent people on the FBI's no-fly and watchlists from purchasing a firearm.

Democrats have been promised a vote on the measure, and are currently in the process of trying to build enough bipartisan support to clear a 60-vote threshold.

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Source: Agencies