WASHINGTON - Hundreds of demonstrators were rallying in Washington, D.C., and across the United States on Saturday, calling on lawmakers to pass legislation aimed at curbing gun violence following last month's massacre at a Texas elementary school.

March for Our Lives (MFOL), the gun safety group founded by student survivors of the 2018 massacre at a Parkland, Florida, high school, said it has planned more than 450 rallies for Saturday, including in New York, Los Angeles and Chicago.

In Washington, hundreds of people steadily arrived at the National Mall near the Washington Monument under light rain.

The organization's 2018 march on Washington, weeks after 17 people were killed at Marjory Stoneman Douglas High School in Parkland, brought hundreds of thousands of people to the nation's capital to pressure Congress to take legislative action, though Republican opposition has prevented any new limits on guns from passing the U.S. Senate.

U.S. President Joe Biden, a Democrat who earlier this month urged Congress to ban assault weapons, expand background checks and implement other gun control measures, said he supported Saturday's protests.

"Today, young people around the country once again march with @AMarch4OurLives to call on Congress to pass commonsense gun safety legislation supported by the majority of Americans and gun owners," Biden said in a Twitter post. "I join them by repeating my call to Congress: do something."

Courtney Haggerty, a 41-year-old research librarian from Lawrenceville, New Jersey, traveled to Washington for the rally with her 10-year-old daughter, Cate, and 7-year-old son, Graeme, to demand congressional action.

Haggerty said the December 2012 school shooting at Sandy Hook Elementary School in Newtown, Connecticut, in which a gunman killed 26 people, mostly six- and seven-year-olds, came one day after her daughter's first birthday.

"It left me raw," she said. "I can't believe she's going to be 11 and we're still doing this."

Cate, who is in the fourth grade, said she wanted to attend. "This is not what I would want my kids to have to be living with," she said.

PRESSURE ON POLITICIANS

This year's event in Washington has a simple message to political leaders, according to organizers: Your inaction is killing Americans.

"We will no longer allow you to sit back while people continue to die," Trevon Bosley, an MFOL board member, said in an emailed statement.

A gunman in Uvalde, Texas, killed 19 children and two teachers on May 24, 10 days after another gunman murdered 10 Black people in a Buffalo, New York, grocery store in a racist attack.

The latest mass shootings have added new urgency to the country's ongoing debate over gun violence, though the prospects for federal legislation remain uncertain.

Among other policies, MFOL has called for an assault weapons ban, universal background checks for those trying to purchase guns and a national licensing system, which would register gun owners.

In recent weeks, a bipartisan group of Senate negotiators have vowed to hammer out a deal, though they have yet to reach an agreement. Their effort is focused on relatively modest changes, such as incentivizing states to pass "red flag" laws that allow authorities to keep guns from individuals deemed a danger to others.

The Democratic-controlled U.S. House of Representatives on Wednesday passed a sweeping set of gun safety measures, but the legislation has no chance of advancing in the Senate, where Republicans have opposed gun limits as infringing upon the U.S. Constitution's Second Amendment right to bear arms.

Speakers at the Washington rally are due to include David Hogg and X Gonzalez, Parkland survivors and co-founders of MFOL; Becky Pringle and Randi Weingarten, the presidents of the two largest U.S. teachers unions; and Yolanda King, granddaughter of civil rights leader Martin Luther King, Jr. Members of MFOL have spent the week meeting with lawmakers in Washington to discuss gun violence.

Organizers expect counter-protesters at the demonstrations, but at noon (1600 GMT) in Washington, none had yet arrived.

Jamie Lee, a 24-year-old pollster in Washington, said she lived in Florida at the time of the Parkland shooting.

"This just keeps happening and nothing gets done," she said. "We need to take better care of our kids in our schools. And that doesn't mean more police officers, it means more counselors and more investment put in our students."

(Reporting by Joseph Ax and Ted Hesson; Editing by Aurora Ellis and Daniel Wallis)