Activist Malala Yousafzai and actress Charlize Theron have lent their support to a new campaign demanding action to help girls around the world get an education.
Organisers of the initiative say that around 130 million girls are missing out on school.
The ONE Campaign has launched #GirlsCount - in which people film themselves saying a number between one and 130 million - to highlight the scale of the problem.
Nobel Peace prize winner Miss Yousafzai, who was shot on a school bus in Pakistan by Taliban gunmen after speaking up over girls' education, U2 singer Bono, screen stars Theron, Michael Sheen and David Oyelowo, as well as singer Angelique Kidjo and Facebook chief Sheryl Sandberg are taking part in the count.
The videos, including those of the celebrities, w ill be combined into "the world's longest ever film to urge world leaders to take the action needed to ensure every girl receives a quality education".
Ms Sandberg, Facebook chief operating officer, said: "130 million girls around the world are out of school. One of the challenges in effectively addressing this global crisis is conveying its staggering scale.
"130 million girls are being denied the basic human right of an education - the domino effect of which none of us can afford to ignore.
"I joined the count at ONE.org, choosing number five, because that's the age millions of girls around the world should be walking into a classroom for the first time. Far too many of them will never get that chance, unless we demand world leaders act."
ONE Campaign's chief marketing officer Roxane Philson said: "It costs less than a loaf of bread to send a girl in one of the world's poorest countries to school each day - a small investment that could save more than a million lives and add more than 100 billion dollars to the global economy every year.
"#GirlsCount is uniting people to make the scale of this crisis clear and to call on leaders to do more."
On International Women's Day on March 8, activists around the world will take part in a global "walk-in", delivering a 340,000-signature petition on the issue to their elected representatives.