Hundreds of thousands of Americans are gathering for nationwide rallies in support of tighter gun control.
March For Our Lives grew out of a movement calling for change after 17 people were killed by a gunman at a high school in Florida last month.
On Friday, the White House said it would ban bump stocks, which give semi-automatic guns rapid-fire capability.
But many activists are calling for more extensive legislation.
The main march is taking place in Washington DC, with more than 800 sister protests taking place across the country and around the world, including London, Edinburgh, Geneva and Sydney.
Organisers have said it may become “standing room only” in Washington, as the swelling crowds could make it hard to move. The march there begins at 12:00 local time (16:00 GMT).
Participants want to seize on public outrage in the wake of the 14 February massacre at Marjory Stoneman Douglas High School in Parkland to convince US politicians to finally take decisive action, including by banning the sale of assault weapons.
However, the issue divides Americans. The right to bear arms is protected under the 2nd amendment of the US constitution and the National Rifle Association (NRA) gun lobby remains highly influential.