The demonstration appeared to the biggest protest on the island in nearly two decades.
"Finally, the government's mask has fallen," said Jannice Rivera, a 43-year-old mechanical engineer who lives in Houston but was born and raised in Puerto Rico and flew in solely to join the protest.
The protest came 10 days after the leak of 889 pages of online chats in which Rosselló and some of his close aides insulted women and mocked constituents, including victims of Hurricane Maria.
The leak has intensified long-smoldering anger in the U.S. territory over persistent corruption and mismanagement by the island's two main political parties, a severe debt crisis, a sickly economy and a slow recovery from Maria, which devastated Puerto Rico in September 2017.
"The people have awakened after so much outrage," said 69-year-old retired nurse Benedicta Villegas. "There are still people without roofs and highways without lights. The chat was the tip of the iceberg."
The crowd surged along the American Expressway despite the punishing heat — toddlers, teenagers, professionals and the elderly, all dripping in sweat and smiling as they waved Puerto Rico flags large and small and hoisted signs.
One group dragged a portable karaoke machine and chanted, "Ricky, resign!"
"This is to show that the people respect themselves," said Ana Carrasquillo, 26. "We've put up with corruption for so many years."
Rosselló, a Democrat, announced Sunday evening that he would not quit, but sought to calm the unrest by promising not to seek re-election in 2020 or continue as head of his pro-statehood political party. That only further angered his critics, who have mounted street demonstrations for more than a week.
"The people are not going to go away," said Johanna Soto, of the city of Carolina. "That's what he's hoping for, but we outnumber him."
The territory's largest newspaper, El Nuevo Dia, added to the pressure with the front-page headline: "Governor, it's time to listen to the people: You have to resign."
Rosselló's secretary of public affairs, Anthony Maceira, said Monday morning he didn't knew Rosselló's whereabouts. Asked who was advising Rosselló on staying in office, Maceira said the governor was speaking with his family, and "that carries a great weight." Rosselló's father, Pedro, was governor from 1993 to 2001.
Asked whether the governor should step down, President Donald Trump said that Rosselló is a "terrible" governor and that hurricane relief money sent to Puerto Rico has been "squandered, wasted and stolen" and the island's top leadership is "totally, grossly incompetent."
Organizers labeled the planned road shutdown "660,510 + 1" — which represents the number of people who voted for Rosselló plus one more — to reject his argument that he is not resigning because he was chosen by the people.
In a video posted Sunday night on Facebook, Rosselló said he welcomed people's freedom to express themselves. He also said he is looking forward to defending himself as Puerto Rico's legislature begins exploring impeachment.
"I hear you," he said. "I have made mistakes and I have apologized."
The demonstrations in this territory of more than 3 million American citizens represent the biggest protest movement on the island since Puerto Ricans rallied to put an end to U.S. Navy training on the island of Vieques more than 15 years ago.
Monday was the 10th consecutive day of protests, and more are being called for later in the week. The island's largest mall, Plaza de las Américas, closed ahead of the protest, as did dozens of other businesses. The upheaval also prompted at least four cruise ships to cancel visits to Puerto Rico, and many officials worry about the effect a resignation would have on the already fragile economy.
Puerto Rico is struggling to restructure part of its $70 billion in debt under federal supervision and deal with a 13-year recession through school closings, cutbacks in infrastructure maintenance and other austerity measures.
At the same time, the island is trying to rebuild from Maria, which caused more than $100 billion in damage, threw Puerto Rico into a year-long blackout and left thousands dead, most of them succumbing during the sweltering aftermath.
Also, the island has seen a recent string of arrests of Puerto Rico officials on corruption charges. Those arrested included the former education secretary.