Democratic Party members are staging a sit-in on the floor of the lower house of the US Congress to demand tighter gun controls after the shootings at a gay nightclub in Florida.
One congressman, John Lewis, urged his colleagues to never give up.
The Republican speaker of the House, Paul Ryan, dismissed the protest as a publicity stunt.
Senators are pushing for a compromise, with top Democratic Senator Harry Reid supporting a Republican proposal.
Mr Reid said he supported new legislation proposed by Republican Senator Susan Collins that would stop gun sales to a limited number of people who are on some terrorism watch lists.
The bill is due to come before the Senate on Thursday. "Even though it may be a small step forward, at least it is a step forward," Mr Reid said.
On 12 June, a man claiming allegiance to the so-called Islamic State group, Omar Mateen, killed 49 people at the Pulse club in Orlando, in the deadliest shooting in modern US history.
At the start, it seemed like a spontaneous moment of protest. But now the Democrats on the House floor are playing the long game. Sleeping bags, pillows, blankets and essential donuts have been shipped in to the growing number now occupying the House chamber. Hundreds who have been watching the action online have gathered outside chanting "do your job" in sympathy.
This is unprecedented. Years of frustration at being unable to pass stricter gun control measures have now culminated in political drama. But even if they were given a vote it would most likely not pass. One hundred bills on gun reforms have gone before Congress in the last five years and none of them have been successful. So what will have changed?
This is an election year where Democrats are trying to wrestle back control of the House and the Senate. Perhaps emboldened by recent polls which show most Americans would prefer stricter laws on guns, Democratic representatives are making it clear to the electorate that if you want change, you know which way to vote.
It was one of the most extraordinary scenes ever witnessed in Congress, the BBC's Laura Bicker reports from Washington.
The Republican leadership switched off the House cameras but the protesters took their message online, tweeting pictures and streaming live video on social media.
Hours into the protest, Speaker Paul Ryan tried to resume normal proceedings, but he was shouted down.
Compared to the pressing issue at hand, discussing technology may seem trivial. But the way in which the outside world got to witness the Democrats' sit-in will surely be considered a defining point for live online streaming.
With cameras shut off, US public service broadcaster C-Span resorted to running pictures being broadcast on Periscope, the live-streaming app owned by Twitter. The pictures were being filmed not by some social media intern or techy onlooker, but by Democrat Scott Peters on his smartphone.
Other feeds sprang up via Facebook Live, a competing platform, while CNN even managed to use Apple's FaceTime feature to conduct a live interview with Democratic representative Steve Israel on the House floor (until he was politely asked to move into the corridor).
The floor of the House became chaotic, with Republicans and Democrats shouting at each other over the issue.
The sit-in is being led by Mr Lewis, a veteran of the civil rights movement of the 1960s.
"What has this body done [to respond to the violence]?'' Mr Lewis asked, referring to several failed efforts in the past week to pass a gun control bill.
"Nothing. We have turned a deaf ear to the blood of innocents. We are blind to a crisis. Where is our courage? How many more mothers… and fathers need to shed tears of grief?"
President Barack Obama took to Twitter to thank Mr Lewis "for leading on gun violence where we need it most".
The lawmakers want a vote to be held before the scheduled break at the end of the week, ending on 5 July.
Mr Ryan told CNN he would not bring a gun control vote in the House of Representatives.
"They know that we will not bring a bill that takes away a person's constitutionally guaranteed rights without… due process," he said.