Mayors in the county warned that the beaches were only reopening for activities deemed “essential,” a list that included fishing, surfing and taking care of pets.

“Just to be clear, this is an opportunity for people to come out to the beach and exercise a couple of times a day,” Mayor Charlie Latham of Jacksonville Beach said at a news conference with other local mayors. “It’s not a sunbathing opportunity.”

Organized group activities, such as picnics and team sports, will still be prohibited, and park restrooms will be closed. Beaches will open from 6 a.m. to 11 a.m. and from 5 p.m. to 8 p.m.

“This can be the beginning of the pathway back to normal life,” Mayor Lenny Curry of Jacksonville said in a video address on Thursday, when he announced the reopening. He and other officials pleaded with residents to be careful and patient, and some warned that the privilege could be revoked if proper safety guidelines were not followed.

Asked about crowded Florida beaches, Dr. Deborah L. Birx, the coronavirus response coordinator, indicated she would defer to county health officials.

“I’d have to link that with a specific county and look at their case rates,” she said at a White House news briefing. “If the county health directors believe that that’s appropriate for their county, then I’m not going to second-judge an individual’s approach to this.”

Mr. Trump chimed in, saying: “Many of the counties, as you know, are really free of this horrible enemy. So we’re opening up. You’ll be seeing a lot of this country start to open up fairly quickly.”