Hurricane Irma is roaring along a path pointing to Puerto Rico, the Dominican Republic, Haiti and Cuba before possibly heading for Florida over the weekend.
The eye of the storm passed over Barbuda at around 1.47am, the US National Weather Service said. Residents said over local radio that phone lines went down.
Heavy rain and howling winds raked the neighbouring island of Antigua, sending debris flying as people huddled in their homes or government shelters.
Officials warned people to seek protection from Irma's "onslaught" in a statement that closed with "May God protect us all".
In Barbuda, the storm ripped the roof off the island's police station forcing officers to seek refuge in the nearby fire station and at the community centre that served as an official shelter. The Category 5 storm also knocked out communication between islands.
Speaking from Antigua, Gemma Handy, a former Express & Star reporter, said this morning: "We're broadcasting live from the local radio station and we've been broadcasting live for 12.5 hours, taking constant calls from the public, trying to reassure people giving them constant weather updates.
"Right now the wind speeds are 75-90mph we can expect those to pick up to around 150mph in the next hour or so and hopefully they they should lessen in a couple of hours.
The former Shrewsbury High School student told the BBC: "We are getting reports that several roofs have sadly been blown off, including, we hear, an entire apartment building roof in one area of Antigua.
"The biggest cause of concern right now is that we seem to have completely lost concern with our sister island of Barbuda.
"They are getting real the full force of this right now they are currently in the eye that should be coming to a close soon and then they are going to got those 185mph winds. We'll be very grateful when we finally get some news back from Barbuda.
"The advice has been that if you're in a flood-prone area: get out, no two ways about it. Obviously those big storm surges are quite dramatic.
"There's been some very strong encouragement. People are used to being in a hurricane area, most people have gone to shelters.
"There are 43 of them across the island. We're hearing as many as 200 people are in some, which for Antigua is quite unprecedented."
"There's definitely a sense that this is like nothing they've ever experienced. The noises outside just go way beyond what that a category 4 hurricane sounded like, so yes it's certainly dramatic.
"People here tend to take things in their stride quite well. Some people are calling in a little panicky, other people much more gung ho about it and even sharing some light-hearted stories.
"We're quite lucky because we're in what we've described as a bit of a bunker because we're at the station and it's a very sturdy concrete building.
"Other people have reported buildings shaking, people are ringing up saying they feel like roof is coming off, so very dramatic conditions.
"We feel a little bit sheltered from it here. Because we're trying very hard to keep other people calm, it's helping keep us calm too."
Midcie Francis of the National Office of Disaster Services confirmed there was damage to several homes but said it was too early to assess the extent of damage.
The storm had maximum sustained winds of 185mph, according to the Hurricane Centre. It said winds would fluctuate slightly, but the storm would remain at Category 4 or 5 strength for the next day or two.
The most dangerous winds, usually nearest to the eye, were forecast to pass near the northern Virgin Islands and near or just north of Puerto Rico on Wednesday.
"I hear it's a Cat 5 now and I'm terrified," Antigua resident Carol Joseph said as she finished her last trip to the supermarket before seeking shelter. "I had to come back for more batteries because I don't know how long the current will be off."
On the 108-square-mile island, people who live in low-lying areas were staying with friends and relatives on higher ground or sleeping in churches, schools and community facilities built to withstand hurricanes. None of the shelters had yet been tested by Category 5 winds, however.
Many homes in Antigua and Barbuda are not built on concrete foundations or have poorly constructed wooden roofs.
President Donald Trump declared emergencies in Florida, Puerto Rico and the US Virgin Islands, and authorities in the Bahamas said they would evacuate six southern islands.
The northern Leeward Islands were expected to see normal tide levels rise by as much as 11ft while the Turks and Caicos Islands and south-eastern Bahamas could see a surge of 20ft and higher waves later in the week, forecasters said.
Bahamas Prime Minister Hubert Minnis said his government was evacuating six islands in the south because authorities would not be able to help anyone caught in the "potentially catastrophic" wind, flooding and storm surge. People there would be flown to Nassau in what he called the largest storm evacuation in the country's history.
"The price you may pay for not evacuating is your life or serious physical harm," Mr Minnis said.
The US National Weather Service said Puerto Rico had not seen a hurricane of Irma's magnitude since Hurricane San Felipe in 1928, which killed a total of 2,748 people in Guadeloupe, Puerto Rico and Florida.
"The dangerousness of this event is like nothing we've ever seen," Puerto Rico Governor Ricardo Rossello said. "A lot of infrastructure won't be able to withstand this kind of force."
The director of the island's power company has warned that storm damage could leave some areas without electricity for about a week to as long as six months.
The eye of the storm was expected to rip westward on a path taking it a little north of Puerto Rico, the Dominican Republic, Haiti and Cuba.
In Florida, people stocked up on drinking water and other supplies.
Florida Governor Rick Scott activated 100 members of the Florida National Guard to be deployed across the state, and 7,000 National Guard members were to report for duty on Friday when the storm could be approaching the area. On Monday, Mr Scott declared a state of emergency in all of Florida's 67 counties.
Officials in the Florida Keys geared up to get tourists and residents out of Irma's path, and the mayor of Miami-Dade County said people should be prepared to evacuate Miami Beach and most coastal areas.
Mayor Carlos Gimenez said the voluntary evacuations could begin as soon as Wednesday evening. He activated the emergency operation centre and urged residents to have three days' worth of food and water.