Hundreds of thousands of Americans galvanized by last month's Florida school massacre rallied on Saturday at nationwide March For Our Lives protests led by students demanding tighter gun laws.

Carrying signs with slogans including "If they choose guns over our kids, vote them out," protesters in Washington jammed Pennsylvania Avenue as students from the Parkland, Florida, high school where 17 people were killed called on lawmakers and President Donald Trump to confront the issue.

The massive rallies aim to break a legislative gridlock that has long stymied efforts to increase restrictions on firearms sales in a nation where mass shootings like the one on Feb. 14 at Marjory Stoneman Douglas High School have become frighteningly common.

"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 the sun was shining on 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," he said, pointing at the white-domed Capitol. "We can and we will change the world!"

Television images showed youthful marchers filling streets in cities across the United States including Atlanta, Baltimore, Boston, Chicago, Los Angeles, Miami, Minneapolis and New York.

More than 800 demonstrations were set to take place in the United States and around the world, according to coordinators, with sister events taking place as far afield as London, Mauritius and Stockholm.

Organizers want the U.S. Congress, many of whose members are up for re-election in November, 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.

In Los Angeles, marcher Carly Dutcher, a high school freshman from Studio City, California, said she was shocked when a theatre teacher showed her and her classmates how they could pull a tampon dispenser off a bathroom wall and use it to battle a school shooter.

"Regardless if I can vote or not, it (gun violence) is the most important issue affecting me and the most important issue in the country right now," Dutcher said.

THIS IS THE NORM FOR US

At a rally near New York`s Central Park, a moment of silence was held for the Parkland victims. The accused gunman, 19-year-old former Marjory Stoneman student Nikolas Cruz, faces the death penalty if convicted on multiple murder charges.

Pop star Paul McCartney was among the marchers, telling CNN he had a personal stake in gun control. "One of my best friends was shot not far from here," he said, referring to Beatles bandmate John Lennon, who was gunned down near the park in 1980.

Jack Graziano, a 16-year-old junior from Long Island`s Massapequa High School, said he wanted gun violence to end so his own children would not have to suffer it.

"Anybody who`s in high school right now has been growing up on school violence," he said. "As awful as it sounds, this is the norm for us."

Taking aim at the National Rifle Association gun lobby, teenagers chanted, "Hey, hey, NRA, how many kids have you killed today?"

The youthful U.S. organizers have won kudos and cash from dozens of celebrities, with singer Demi Lovato 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 attorney wife, Amal, have donated $500,000 and said they would be at the Washington rally.

On Friday, Trump signed a $1.3 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 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.

Former president Barack Obama said on Twitter that he and his wife Michelle were inspired by all the young people who made the marches happen.

"Keep at it. You're leading us forward. Nothing can stand in the way of millions of voices calling for change," Obama said.