GLASGOW—Organizers of the COP26 United Nations local climate summit welcomed one of their maximum-profile critics Friday: Greta Thunberg.

Ms. Thunberg is headlining an hourslong protest march and rally in Glasgow that kicked off early Friday. Hundreds of generally younger protesters, carrying colorful banners and occasionally accompanied by their mom and dad, marched from Glasgow’s Kelvingrove Park to the metropolis middle.