Chaos in the US House of Representatives stretched into Thursday after Republicans refused demands by Democrats, who staged a "sit-in", to bring up gun control legislation in the wake of the nation's worst mass shooting in modern history.

Scores of Democrats flooded the chamber for more than 14 hours, sitting on the floor and chanting in a "sit-in" protest, taking over the House even as the Republican leadership shut off television cameras and microphones as they tried to force lawmakers back to order.

Democrats vowed to continue their protest until Republican leaders allowed a vote on gun control legislation after the June 12 massacre in Orlando, Florida. Republicans called their move a publicity stunt.

"Mr. Speaker, this is not a publicity stunt. This is a wakeup call," Representative Mark Takano, a California Democrat, said on the House floor.

Republican House Speaker Paul Ryan refused Democrats' demands for action on gun control and instead forced a vote on an unrelated bill even as raucous scenes spilled across the legislative chamber.

Later, the House also voted to adjourn, closing out Wednesday's "legislative" day, but planned to return at 2:30 a.m. EST (0630 GMT) to hold more votes.

During the protest, several Republican representatives charged the chamber floor and yelled at Democrats, prompting a tense confrontation that nearly descended into fisticuffs. Oklahoma Republican Markwayne Mullin of Oklahoma, a former mixed martial arts fighter, made his way through an aisle thick with members in an apparent attempt to ease tensions.

It was a new low for the House, which in recent years has become mired in partisanship and legislative deadlock.

"This is not a great moment for the House of Representatives," Brad Sherman, another California Democrat, said amid the melee. "Order has broken down because we have a speaker who has not allowed us to vote."

Ryan called for decorum but could scarcely be heard over Democrats chanting "no bill, no break!" and insisting that the chamber remain in session despite a week-long break scheduled to start this weekend.

The Democrats began their protest shortly before midday (1600 GMT) Wednesday, bringing other House business to a halt as they occupied the floor. Ryan entered the chamber about 10 hours later and announced an immediate vote on an unrelated investment advice bill.

Democrats held up signs honoring gun violence victims during the vote.

"We will stay," House Democratic leader Nancy Pelosi said on the chamber floor as members vowed to continue their sit-in as long as it took to get a vote on a gun bill.

Ryan insisted he would not bring up any bill that would take away gun owners' constitutional rights.

The Democrats' move echoed last week's filibuster by Senate Democrats to protest against inaction on guns in the wake of the massacre at a gay nightclub in Orlando, Florida, where a gunman killed 49 people and wounded 53.

After the Senate talk-a-thon, the Senate's Republican majority scheduled votes on four gun control measures - all of which failed on Monday. Work on a compromise is under way.

Guns are a potent US political issue and Americans are on edge after mass shootings in recent years in Connecticut, Colorado, California and elsewhere. The House chaos reflected what is fast becoming a heated issue in the November 8 US presidential election.

Congress has not passed major gun control legislation since 1994, with gun rights defenders saying such measures infringe on the constitutional right to bear arms.

Ryan also remained steadfast in not allowing a vote on the issue. Instead, members voted on the investment bill and moved to advance a $1.1 billion funding measure to fight the Zika virus.

Led by Representative John Lewis, a Democrat from Georgia and veteran of the 1960s civil rights movement, Democrats urged tighter background checks and legislation to curb the sale of weapons to people on government watch lists.

Lewis made an impassioned speech on the House floor to the more than 100 fellow Democrats huddled around him urging them to "never, ever give up."

"They (the American public) want us to do something. We have a moral obligation, a mission and a mandate to do something," Lewis said to applause and cheering.

Pelosi invoked not only Orlando but other mass shootings, including an attack a year ago by a white man at a black church in Charleston, South Carolina, that killed nine. "Right now there is an opportunity," she said.

Several Democratic senators crossed the Capitol on Wednesday to join protesters, including Senators Elizabeth Warren, Tim Kaine and Cory Booker, all mentioned as potential running mates for presumptive Democratic presidential nominee Hillary Clinton, who voiced her support on Twitter.

Clinton's rival in the Democratic race, Senator Bernie Sanders, also appeared.

"Thank you John Lewis for leading on gun violence where we need it most," President Barack Obama tweeted.

Lawmakers also took to social media to document their demonstration with video and pictures.

Outside the Capitol, nearly 50 people gathered in solidarity at a rally organized by Everytown for Gun Safety, the advocacy group backed by former New York Mayor Michael Bloomberg.

Many House Republicans said they viewed the problem differently from Democrats. "We don't view the fact that someone becomes radicalized and decides to kill a bunch of Americans ... as a gun problem," John Fleming of Louisiana said on Wednesday.

"We view that as a terrorist problem."-Reuters