Police said that in central Leuven alone more than 10,000 students gathered and authorities had to change the planned route to accommodate the marchers. Many thousands more protested in the capital Brussels and other provincial towns throughout Belgium as the youth movement spread further across the country.
The protests have kept a focus on climate change as a political pressure point before national and European Union elections, after 70,000 demonstrators held a climate march through Brussels last month.
Thousands of students are skipping classes to join a march in support of more ambitious climate policies in the Netherlands.
The demonstration Thursday by Dutch students follows similar marches in recent weeks in neighboring Belgium that have drawn thousands of protesters.
Organizers say they want to send a wake-up call to politicians in the Netherlands who are wrestling with how best to rein in greenhouse gas emissions.
The Dutch Environmental Assessment Agency said in a report last month a court-set target of reducing emissions by 25 percent from 1990 levels by 2020 is "out of reach."
A group of about 350 scientists and researchers published an open letter in support of the march, saying it is "high time for tough measures to quickly and drastically reduce greenhouse gas emissions."