Thousands of students streamed out of schools across Europe on Friday, waving placards and carrying banners as they marched as part of a coordinated walkout to demand action on climate change.

In London, students chanting, “Save our planet!” gathered in Parliament Square, where they brought traffic to a standstill. Others held signs that read, “Change the politics not the climate.”

About 200 students gathered outside the Ministry of Ecology in Paris, saying they hoped to repeat the demonstration every week until their demands were heard. They urged the government to reduce France’s greenhouse gas emissions by at least 4 percent per year.