The event is being held to stand with thousands of protestors in Puerto Rico demanding Ricardo Rosselló's resignation.
The latest demonstration came 10 days after the leak of 889 pages of obscenity-laced online chats between Rossello, a Democrat, and some of his close advisers.
"We don't want future generations to suffer what we have suffered", said Tania Garcia, a 25-year-old engineer from the eastern town of Juncos who vowed to keep protesting.
Puerto Rico was already in dire straits even before Hurricane Maria, which laid bare the disastrous state of its electrical grid.
"They can't deny it: The power is in the street", San Juan Mayor Carmen Yulin Cruz wrote in a Twitter message on Monday.
Just as they are on the island, many demonstrators have been protesting out on the streets of South Florida over the past week.
Rossello said Sunday he would face any proceedings "in a responsible manner". And I'll tell you, we had tremendous successes.
The crowd of demonstrators, many waving Puerto Rican flags, largely dissipated after police formed a line and fired the canisters at them, according to Fox News reporter Jeff Paul.
Boris Johnson set to be new prime minister after Conservative party vote
May will leave office on Wednesday after going to Buckingham Palace to see Queen Elizabeth, who will formally appoint Johnson. Boris Johnson will become the UK's next prime minister after securing a landslide victory in the Tory leadership contest.
Crowds of people, some waving the USA island territory's flag, gathered near San Juan's baseball stadium for the latest in a series of more than a week of protests in the capital and elsewhere.
Rossello's announcement on Sunday that he would not seek re-election next year and would step down as head of the New Progressive Party seemed to have little effect on the crowds, who called for him to immediately surrender the governorship.
Demonstrations are also expected to spread to New York City, where protesters are planning to gather in Columbus Circle in Manhattan on Monday afternoon to join the calls for Rossello's resignation.
"That way, I can focus on the job at hand", he said, referring in part to fighting corruption and overseeing recovery efforts following Hurricane Maria.
Budget cuts - which, among other actions, forced the closure of schools - prompted many Puerto Ricans to flee the island. "I have made mistakes and I have apologized".
But Puerto Ricans remained unsatisfied and vowed to keep protesting until he steps down, no matter how long it takes.
Combined with the post-hurricane exodus from the island, Puerto Rico lost four percent of its population.
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