DAVOS, Switzerland — Greta Thunberg, the 17-year-old Swedish climate activist, spent this week inside the halls of power at the World Economic Forum, addressing the titans of global business and politics, huddling with a few of them for closed-door sessions and being publicly belittled by officials from the United States government.

On Friday, as the five-day conclave came to a close, she returned to her original domain: a protest outdoors. Shoulder to shoulder with her youth activist peers, Ms. Thunberg marched along the narrow road that winds through this village in the Swiss Alps, holding her signature handmade black-and-white sign that read, in her native Swedish, “school strike for the climate.”

Speaking to reporters just before the march set off, Ms. Thunberg and four youth activists from Europe and Africa rebuked business and government leaders at the World Economic Forum for not taking climate action and warned that they would continue to press them to stop investing in fossil fuels. Those demands, Ms. Thunberg noted, “have been completely ignored.”

Asked about a suggestion by Treasury Secretary Steven Mnuchin that she learn economics in college before calling for divestment, Ms. Thunberg brushed off such criticisms as irrelevant. “If we care about that, we wouldn’t be able to do what we do,” she said.