He violated a travel ban when he left Venezuela, and international watchers didn't know what would happen when he tried to return. Guaidó declared himself Venezuela's interim president, and he's received wide support from other countries, including the U.S. But current president Nicolás Maduro refuses to relinquish power.
He was sworn in for a second term earlier this year, despite accusations that it was a fraudulent election.
Guaidó tweeted after the rally Monday that there's a lot of work the country needs to do, and there's nothing to celebrate just yet.
He called on the country to mobilize again on Saturday. Additional reporting from Newsy affiliate CNN .