Activists at gun control marches across America have signed up thousands of first-time voters pledging to eject lawmakers who oppose stricter firearms laws.

Voter registration advocates were out in force as hundreds of thousands of people took part in more than 800 March for Our Lives rallies held across the US on Saturday.

Organisers of the drive were aiming to sign up 25,000 young voters in favour of tougher gun control laws ahead of mid-term Congressional elections in eight months’ time.

Speaking at a rally in Washington, Delaney Tarr, a survivor of the Marjory Stoneman Douglas High School shooting in Parkland, Florida, that left 17 people dead in February, had a simple message for legislators.

"If there is no assault weapons ban passed, then we will vote them out," she said.

Volunteers from electoral registration campaign HeadCount chanted "demonstration without registration leads to frustration" before working through the crowds, signing up new voters en masse at the Washington rally.

'No more': Watch David Hogg's incredible March For Our Lives Speech in full

One of those who registered, 18-year-old Hugh Williams from Ypsilanti, Michigan, said gun violence had become a “menace” in his community.

"The more I see people marching and standing up for a cause I believe in, I want my voice to matter, too," he said.

March for Our Lives – in pictures Show all 13 1 /13 March for Our Lives – in pictures March for Our Lives – in pictures Demonstrators chant during the protest for gun legislation and school safety AP March for Our Lives – in pictures Trevon ‘Tre’ Bosley, 19, of Chicago, the brother of Terrell Bosley who was killed in 2006 in a case of mistaken identity, speaks during the rally AP March for Our Lives – in pictures Marjory Stoneman Douglas High School student Delaney Tarr speaks at the rally AFP/Getty March for Our Lives – in pictures Protestors line the streets in Washington Getty Images March for Our Lives – in pictures People arrive for the March For Our Lives rally against gun violence in Washington, DC AFP/Getty March for Our Lives – in pictures Protestors carrying placards in Washington AFP/Getty March for Our Lives – in pictures Protestors hold up placards in Washington AFP/Getty March for Our Lives – in pictures Schoolchildren wear targets ahead of the rally Getty March for Our Lives – in pictures Attendees congregate in preparation for the march Getty March for Our Lives – in pictures Students brought a host of innovative placards AFP/Getty March for Our Lives – in pictures The movement’s main demand is the banning of assault rifles Getty March for Our Lives – in pictures Since the Florida shooting, students have called for urgent gun reform AFP/Getty March for Our Lives – in pictures In the wake of the Florida attack, President Donald Trump called for teachers to be armed AFP/Getty

Participation in US elections by young voters is often low, and registration activists are targeting the four million Americans turning 18 this year, as well as those aged 19 to 21 who have never voted before.

Any potential increase in young voters is likely to be a boon for Democratic and independent candidates who back stricter controls on assault-style weapons, bump stocks and tighter access to guns for young people and the mentally ill.

March For Our Lives: Emma Gonzalez's 6 minute and 20 second speech for the Parkland victims

In Parkland, Sari Kaufman, another Marjory Stoneman Douglas student, urged her peers to get involved in politics in order to remove lawmakers who did not protect their interests.

"With this movement, we will ensure record-breaking turnout not just in the next presidential election, not in the next midterm election, but in all elections," she told a crowd of thousands.

"We're here today to give you the tools to make a change."