The march is building on a "historic surge" of student protests and strikes for climate action, but adults also are formally invited to participate, according to the organizers behind the movement.
The strike on Friday is expected to include more than 1,000 locations across the U.S. and 4,500 worldwide, with more than 2,000 scientists pledging to attend as well, organizers said.
Massive crowds were seen in Japan, Australia, England and New York, with streets packed wall-to-wall with protesters.
In London, many of the young speakers talked about “taking action” and phasing out fossil fuels by 2030. Large police presences have been reported near the various crowds, but the gatherings appeared largely peaceful.
The crowds appeared to be made up of a majority of young people. ABC News reporters spoke to one 9-year-old student in London who wrote a letter to her headmaster explaining why it was important for her to be at the protest on Friday. That young girl was accompanied by her mother, who had taken the day off work to attend with her daughter.
The lead organizer for the New York City march, 19-year-old Shiv Soin, told ABC News that more than 15,000 people have said they're interested in attending on the event's Facebook page.
The youth activists are demanding world leaders stop using fossil fuels, transition to a green economy and hold polluters accountable, Soin said.
"We're going to hold them accountable," he said.
Soin, a politics and economics major at New York University, became passionate about the climate fight after returning to New Delhi in 2011 to attend his grandmother's funeral, where he said he was hospitalized due to the poor air quality.
"I ask myself, will this matter in 20, 30 years if we don't have a livable planet?" he said. "The answer is no."
Demonstrators in New York will meet at a rally in Foley Square and march south to Battery Park, where 16-year-old Swedish climate activist Greta Thunberg will be among the speakers, Soin said.
On Wednesday Thunberg implored U.S. lawmakers at the House Foreign Affairs Committee to "listen to the science" and take "real action" to curb carbon emissions.
New York City Public Schools will be excusing 1.1 million students from class on Friday -- with parental permission -- so they can participate in the strike.
Activists began posted photos of the posters created for the strike earlier in the week.
"'We vote next,' is what we have to say," Soin said.