Thousands of people are expected to fill the streets of San Juan on Monday morning in massive demonstrations to demand that the U.S. territory's governor resign over offensive chat messages.
The Washington Post reported late Sunday that organizers expected more than 1 million residents, about a third of the population, to join in the protests.
There is also a call for a general strike from trade unions. Many business owners decided to close shops and offices for the day, media reported.
Puerto Rico's governor, Ricardo Rossello, 40, on Sunday said he would not seek re-election next year but refused to resign.
Rossello asked for forgiveness and said he respected the wishes of Puerto Ricans.
He said he would not seek a second term in the November 2020 election. He also said he would resign as head of his New Progressive Party (PNP).
"I know that apologizing is not enough," Rossello said in a video posted on Facebook. "A significant sector of the population has been protesting for days. I'm aware of the dissatisfaction and discomfort they feel. Only my work will help restore the trust of these sectors."
His comments drew outrage from many Puerto Ricans, with social media videos showing San Juan residents leaning out of apartment windows banging pots and pans.
The publication on July 13 of sexist and homophobic chat messages between Rossello and top aides unleashed simmering resentment over his handling of devastating hurricanes in 2017, alleged corruption in his administration, and the island's bankruptcy process.
"'#Resign Ricky isn't just a call for him to resign from the party, but from his seat as the top official," tweeted Linda Michelle, an industrial engineer and Puerto Rico radio personality. "Whoever wasn't sure about going to the march tomorrow has now made up their mind to go."
Puerto Rico's non-voting representative to the U.S. Congress as well as Democratic presidential candidates and lawmakers have called for the governor to step aside after nine days of sometimes violent protests.
"Once again: Rosselló must resign," U.S. Representative Alexandria Ocasio-Cortez tweeted on Sunday in response to his video.
But Puerto Rico's Senate President Thomas Rivera Schatz, who may take over leadership of the pro-statehood PNP, said Rossello's actions "put an end to part of the controversies and trauma hitting our people".
Puerto Rico House Speaker Carlos Mendez, also of the PNP, appointed an independent panel on Friday to investigate whether the chats warranted impeachment.
"I welcome the process started by the legislative assembly, which I will confront with complete truth," Rossello said in the video.
The political turmoil comes at a critical stage in the island's bankruptcy process as it tries to restructure some $120 billion in debt and pension obligations.
It has also raised concerns among U.S. lawmakers who are weighing the island's requests for billions of federal dollars for healthcare and work to recover from Hurricane Maria, which led to nearly 3,000 deaths.
Opposing Rossello are a raft of Puerto Rican celebrities ranging from singer Ricky Martin and rapper Bad Bunny to "Hamilton" creator Lin-Manuel Miranda.
Martin, a target of the governor's chats, said he would march with protestors on Monday.
"I want to feel the power of the people," Martin, 47, said in a Facebook video, urging legislative leaders to start an impeachment process.