The "March for Our Lives" is scheduled to take place on March 24, with sister rallies planned across the country. It comes after a 19-year-old armed with a semi-automatic rifle killed 17 people at a high school in Parkland, Florida last week.
Surviving students, who have grown up with mass shootings at US schools but are young enough to feel optimistic that change can come, have vowed to make the tragedy a turning point in America's deadlocked debate on gun control.
"Our family will be there on March 24 to stand side by side with this incredible generation of young people from all over the country," he added.
"In the name of our children Ella and Alexander, we're donating USD 500,000 to help pay for this groundbreaking event. Our children's lives depend on it."
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The pledge from the Clooneys, one of the biggest A-list couples on the planet, who announced the birth of their twins in London last June, follows a slew of other celebrities calling for greater gun controls since the Florida shooting.
The White House says President Donald Trump is supportive of efforts to improve background checks for gun purchases, but many Democrats want far more deep-seated reforms.