BUDAPEST, Hungary—About 10,000 Hungarians marched through central Budapest on Sunday for a fourth day of protests against Prime Minister Viktor Orban, one of Europe’s most outspoken nationalist leaders, and a law that would require them to work up to 400 extra hours a year.

The protesters blew horns and carried trade union banners past hundreds of police, demanding Mr. Orban reverse what his critics have dubbed the “Slave Law.” The law, passed Wednesday, would allow employers to require paid overtime amounting to an extra...