BRUSSELS — Tens of thousands of children skipped school in Belgium on Thursday to join demonstrations for action against climate change, part of a broader environmental protest movement across Europe that has gathered force over the past several weeks.

In Germany, France, Sweden, Switzerland and elsewhere, activists have come together on social media to gather in large numbers and without much apparent preparation, the protests taking a different shape in each country.

In Germany, students have protested on Fridays, communicating mainly through the messaging app WhatsApp; in Belgium, they organize on Facebook and have skipped school by the thousands on four consecutive Thursdays.

Last Sunday, climate protests in Brussels swelled to an estimated 100,000 people of all ages. That same day, an estimated 80,000 took part in cities across France — more than turned out for the “Yellow Vest” protests the day before.