Among those braced for the onslaught was Sally Carlson from St Petersburg in Tampa Bay.
The area, which has a population of about three million, has not taken a direct hit from a major hurricane since 1921.
Ms Carlson said: “I’ve been here with other storms, other hurricanes. But this one scares me. Let’s just say a prayer we hope we make it through.”
Meanwhile, Allison McCarthy Cruse took shelter with seven other adults, three children and seven dogs in a neighbour’s home just a few streets from the water in St Petersburg.
She said she feared the roof on her own house might not survive, adding: “I am prepared to say goodbye to my things, and that is hard.”
John Huston has been riding out the storm in his house on Key Largo in the northern Florida Keys.
The Keys were the first part of Florida to be hit by Irma when, as a category four storm with 130mph winds, it made landfall at Cudjoe Key.
He said he witnessed furniture and fridges floating down the street along with small boats.
He described whiteout conditions, with howling winds that sucked the water from the gulf side of the narrow island, where the tide is usually 8ft deep.
After striking the Keys, the centre of the hurricane, known as the eye, made its way up the southwestern coast of Florida.
On Marco Island, Kathleen Turner and her husband rode out the storm on the second floor of a friend’s property after they were unable to find a flight out.
She said she feared for her home.
“I’m feeling better than being in my house, but I’m worried about my home, about what’s going to happen,” she said.
Miami, on the southeastern coast, was spared the head-on blow it was expecting but still suffered flooding and strong winds.
Although Irma has weakened from a category four to a category two storm, “life-threatening” winds of around 100mph and powerful storm surges have continued to batter the region.
Heavy rainfall and flooding was expected for much of Florida and parts of the southeast US over the coming days.
At least three people have died in the state and more than three million have been left without power.