The People's Climate March is campaigning for curbs on carbon emissions, ahead of the UN climate summit in New York next week.
In Manhattan, organisers said some 310,000 people joined a march that was also attended by UN chief Ban Ki-moon.
Earlier, huge demonstrations took place in Australia and Europe.
"This is the planet where our subsequent generations will live," Mr Ban told reporters. "There is no 'Plan B' because we do not have 'Planet B'."
The UN Secretary General was accompanied by primatologist Jane Goodall and the French Ecology Minister, Segolene Royal.
New York hosted the largest of Sunday's protests, drawing more than half of the 600,000 marchers estimated by organisers to have taken part in rallies around the world.