Using loudspeakers, police in patrol cars ordered people off the beach: "Please stay at home. Governor's executive orders. The beach is closed."
The sweep surprised many tourists in the capital of San Juan, some of whom disregarded orders to stay inside their hotels on a sunny day.
"Why would I get sick at the beach? I'm not going to be touching anything," said 46-year-old David Zimmer of Richmond, Minnesota, as he joined a group of family and friends flip-flopping their way to a beach that police had driven through just an hour ago to empty it out.
Other tourists heeded the warnings and shuffled back to their hotels, many carrying take-out meals before locking themselves in.
Puerto Rico Secretary of State Elmer Román said at a press conference on Monday that no one is allowed to go to the beach, clarifying an executive order issued Sunday that has confused many.
"Unfortunately, tourists have to stay in their rooms," he said.
Román earlier told local residents, "You will stay in your house unless you are going to buy food or medicine," he said. "People have to understand this is something serious."
Puerto Rico had recorded only five confirmed cases of COVID-19 as of Monday, and no deaths.
The new coronavirus causes only mild or moderate symptoms, such as fever and cough, for most people, though it can cause more severe illness for older adults and people with existing health problems.
The World Health Organization says people with mild illness recover in about two weeks, while those with more severe illness may take three to six weeks to recover.
Brad Dean, CEO of Discover Puerto Rico, said in a phone interview that the mandate comes during one of the busiest times of the year and will have a significant impact on tourism, which makes up nearly 7% of the island's economy.
"We understand that some visitors may be disappointed, but the safety of residents and visitors has to be of utmost priority," he said.
Elsewhere across Puerto Rico, authorities said Monday they had fined more than two dozen businesses and arrested a bar owner for violating the curfew, and they warned that violators would be pursued more aggressively in upcoming days.
Officials also announced that the National Guard will start screening passengers arriving at Puerto Rico's main airport by Wednesday and that the government will hold all meetings and press conferences online only.
The situation also led the Federal Emergency Management Agency to announce Monday that it was temporarily suspending operations to help hundreds of Puerto Ricans who lost their home last month during a series of strong earthquakes.
The announcement was followed by some good news: Puerto Rico's government said it would not charge rent at public housing complexes until the curfew is lifted.
Meanwhile, certain private sector employees complained their bosses were ordering them to go to work despite the curfew, prompting the government to urge all non-essential workers to stay at home.
Román warned that police will issue fines to individuals, not just businesses accused of violating the order. Officers have started 12-hour shifts to ensure people comply with the curfew, with violators facing a six-month jail term or a fine of up to $5,000.
On Sunday, Gov. Wanda Vázquez ordered a two-week closure of nonessential government offices and commercial businesses except for gas stations and those in the food, health and finance sectors. Puerto Rico also cancelled major events, including an Ironman race scheduled for this past weekend.
Tizgel High, a 41-year-old athlete from Nashville, Tennessee who was supposed to compete in the Ironman competition, resigned herself on Monday to run around a track near the beach. She had received an alert via her cellphone on Sunday that people were banned from being outside from 9 p.m. to 5 a.m., but she had not heard that they could not be outside during the day.
"Not being on the beach doesn't make much sense to me," she said.