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.

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.”

A crowd much larger than the one which witnessed Donald Trump’s inauguration demands the immediate reform of US gun laws. Photo: ABC/Philip Williams

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,” said Ashley Schlaeger, an 18-year-old freshman at Ohio State University who drove to the Washington rally with friends.

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.

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.

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, Republican President Donald Trump 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,” said Ms Walters, 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.