MELBOURNE, Australia — As wildfires raged across the country, a heat wave again threatened the Great Barrier Reef and a mining giant announced it would push ahead with a huge coal project, thousands of students across Australia quit school for the day on Friday to protest government inaction on climate change.

The students — ranging in age from 5 to 18 — came from more than 200 schools to demand that the federal government block construction of the Adani Carmichael coal mine, block any new coal or gas projects and require 100 percent renewable energy use by 2030.

“I am here because I am terrified,” Harriet O’Shea Carre, 14, told a crowd of thousands of student marchers outside the Old Treasury Building in Melbourne. Protesters spilled out onto the road with signs saying “civil disobedience requires no permission slip” and “don’t be a fossil fool.”

Ms. O’Shea Carre, an organizer of the march and a student from Castlemaine Steiner School in Muckleford, about 70 miles northwest of Melbourne, predicted dire consequences if the government failed to act on climate change.