Tens of thousands of Americans have gathered across the country at March For Our Lives rallies to demand tighter gun laws, led by survivors of last month's Florida school massacre.

Key points: As many as 500,000 people were expected to demonstrate near the Capitol in Washington

As many as 500,000 people were expected to demonstrate near the Capitol in Washington Student survivors of the Parkland, Florida shooting were set to speak in Washington

Student survivors of the Parkland, Florida shooting were set to speak in Washington Organisers want Congress to ban the sale of assault weapons and tighten background checks

Students from the Parkland, Florida, high school where 17 people were killed on February 14 called on politicians and US President Donald Trump to confront the issue.

"Politicians. Either represent the people or get out. Stand with us or beware, the voters are coming," Cameron Kasky, a 17-year-old junior at the high school told the crowd.

Another Parkland survivor, David Hogg, said it was a new day.

"You can hear the people in power shaking," he said to loud applause.

"We're going to make sure the best people get in our elections to run not as politicians, but as Americans. Because this — this is not cutting it."

Hordes of people fill the streets of Washington during a gun control rally. ( ABC News: Philip Williams )

Organisers said as many as 500,000 people were expected to demonstrate near the Capitol in Washington and call on Congress to fight gun violence.

The protests aim to break a legislative gridlock that has long stymied efforts to increase restrictions on firearms sales in a nation where mass shootings at schools and colleges have become a frequent occurrence.

"I don't want any child to be seen as another statistic," Ashley Schlaeger, an 18-year-old freshman at Ohio State University who drove to the Washington rally with friends, said.

Marjory Stoneman Douglas High School student David Hogg salutes the crowd at the Washington rally. ( ABC News: Jonathan Ernst )

Parkland student Emma Gonzalez also addressed the masses.

"Six minutes and about 20 seconds. In a little over six minutes, 17 of our friends were taken from us, 15 were injured and everyone, absolutely everyone in the Douglas community, was forever altered," she said.

"Everyone who has been touched by the cold grip of gun violence understands.

"For us, long, tearful, chaotic hours in the scorching afternoon sun were spent not knowing. No one understood the extent of what had happened. No one could believe that there were bodies in that building waiting to be identified for over a day."

Emma Gonzalez says the shooting changed the lives of everybody in the school community. ( Reuters: Jonathan Ernst )

In the Fort Lauderdale suburb of Parkland, thousands of people passed through police checkpoints to assemble in a park for a rally and march.

Many held signs with slogans including "Am I Next?", "A Call To Arms For the Safety of Our Sons and Daughters", and "Congress = Killers".

Adam Buchwald, who survived the mass shooting at Marjory Stoneman Douglas High School, told the crowd he and his friends would stay focused on getting new legislation passed.

"Sadly, this could be repeated in your city or town. This stops now!" he said to loud cheers.

At a rally in New York, a moment of silence was held for the Parkland victims.

The accused gunman, 19-year-old former student Nikolas Cruz, faces the death penalty if convicted.

Delaney Tarr, a survivor of the Parkland, Florida shooting, speaks during the rally in Washington. ( AP: Andrew Harnik )

Celebrities affected by violence join students

Musicians Paul McCartney and Miley Cyrus and comedian Amy Schumer were among a number of stars who played supporting roles at the nationwide rallies.

The protests were deeply personal for some of the celebrities involved.

Singer Jennifer Hudson, who performed Bob Dylan's The Times They Are A Changin', alluded to the shooting deaths of her mother, brother and seven-year-old nephew in 2008.

"We've all lost somebody … we've all got a purpose. And we want what? We want change," she said, encouraging the vast crowd to join her in song.

Jennifer Hudson alluded to the shooting death of her family members in 2008. ( Reuters: Jonathan Ernst )

McCartney said his decision to take part in the New York City rally was prompted by the 1980 Manhattan shooting death of John Lennon, his former Beatles bandmate.

"One of my best friends was killed in gun violence, right 'round here, so it's important to me," he told CNN.

Asked what he hoped could be accomplished by the event, McCartney opened his jacket to show the slogan emblazoned on his black T-shirt: "We can end gun violence."

Paul McCartney said he joined the rally in memory of the shooting death of John Lennon. ( Reuters: Shannon Stapleton )

Celebrities attending the Washington event included power couples George and Amal Clooney and Kate Capshaw and Steven Spielberg, who helped fund the protests with a combined $1 million ($1.3 million) donation.

Also attending were Kim Kardashian and Kanye West, Glenn Close, Cher, Jimmy Fallon and Dennis Rodman.

Cyrus, Andra Day, Vic Mensa, Demi Lovato, Lin-Manuel Miranda, Ben Platt and Ariana Grande performed in Washington, and Rita Ora took the stage in Los Angeles.

Ariana Grande performed Be Alright during the rally. ( AP: Alex Brandon )

"This song is dedicated to Stephon Clark, Decynthia Clements and all the unarmed black men and women killed by police weapons," Mensa said before performing Now We Could Be Free.

Grande encountered another iteration of violence when her 2017 Manchester, England, concert was bombed, killing 22 people and injuring scores of others.

Rallies held across the world

More than 800 demonstrations were set to take place in US cities from Los Angeles to New York and in countries around the world, according to coordinators.

In Sydney, rally organiser Jennifer Smith told a crowd of about 300 people, many of them Americans, that she could "breathe easy" about sending her children to school in Australia with its tough gun laws.

"I never have to worry about them having to do active shooter drills," she said.

Organisers of the US marches retweeted photos from sister demonstrations as far afield as Northern Ireland, Mauritius, and Stockholm.

Organisers want the US Congress, many of whose members are up for re-election this year, to ban the sale of assault weapons like the one used in the Florida rampage and to tighten background checks for gun buyers.

On the other side of the debate, gun rights advocates cite constitutional guarantees of the right to bear arms.

Houston Police Chief Art Acevedo (centre) and Mayor Sylvester Turner (far right) join demonstrators in Houston. ( AP: David J. Phillip )

The teenage US organisers have won kudos and cash from dozens of celebrities, with singer Ariana Grande and "Hamilton" creator Lin-Manuel Miranda among those performing in Washington.

"Trainwreck" star Amy Schumer is set to appear at the Los Angeles rally.

Actor George Clooney and his human rights barrister wife, Amal, have donated $US500,000 ($650,000) and said they would be at the Washington rally.

On Friday Mr Trump — a Republican — signed a $1.69 trillion spending bill that includes modest improvements to background checks for gun sales and grants to help schools prevent gun violence.

Democrats and nonpartisan groups hope to register at least 25,000 first-time voters at the rallies, potentially a boost for Democrats, who generally favour stricter gun controls.

White House deputy press secretary Lindsay Walters said the administration applauded "the many courageous young Americans" exercising their free-speech rights on Saturday.

"Keeping our children safe is a top priority of the President's," Ms Walters said, noting that on Friday the Justice Department proposed rule changes that would effectively ban "bump stock" devices that let semi-automatic weapons fire like a machine gun.

AP/Reuters