The high school age students who organized the event walked out of classes Friday.
They were bolstered by supporters coming from earlier rallies on the Harvard and MIT campuses and outside the Cambridge offices of Google, Microsoft and Amazon.
The crowds converged on Boston City Hall and later the Massachusetts State House, halting downtown traffic along the way.
Gina McCarthy, who headed up the U.S. Environmental Protection Agency under President Barack Obama, was among the keynote speakers.
Democratic Mayor Marty Walsh also spoke. He estimated about 7,000 people were in attendance.
Organizers want Gov. Charlie Baker to declare a climate emergency. They also want Massachusetts to stop using fossil fuels.
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