Below, the most powerful moments.
Emma Gonzalez's 6 minutes of silence
With tears streaming down her face, 17-year-old Emma Gonzalez stood in silence for 6 minutes and 20 seconds - the amount of time it took a gunman to shoot and kill 17 of her friends and classmates last month.
Naomi Wadler's powerful speech
11-year-old Naomi Wadler took to the stage to represent all of the African-American victims of gun violence. “I represent the African-American women who are victims of gun violence, who are simply statistics instead of vibrant, beautiful girls full of potential,” Wadler said. “For far too long, these black girls and women have been just numbers. I am here to say never again for those girls too.”
Paul McCartney walking in honour of John Lennon
Paul McCartney joined protesters in New York to honour his Beatles bandmate and gun violence victim John Lennon.
Celebrities such as George and Amal Clooney, Reese Witherspoon, Laura Dern, Jennifer Hudson, Ariana Grande, Kim Kardashian, Demi Lovato, Selena Gomez and Miley Cyrus also attended marches.
Martin Luther King's granddaughter
Martin Luther King's nine-year-old granddaughter, Yolanda Renee King, held the hand of Parkland survivor Jaclyn Corin as she told crowds, "My grandfather had a dream that his four little children will not be judged by the colour of their skin but by the content of their character.
I have a dream that enough is enough. That this should be a gun-free world. Period.”