Officials earlier warned people to seek protection from Irma's "onslaught" in a statement that closed with: "May God protect us all."
Heavy rain and howling wind raked the island of Antigua as Irma arrived, sending debris flying as people huddled in their homes and government shelters.
The National Hurricane Centre's forecast was for the winds to fluctuate slightly but for the storm to 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.
The northern Leeward Islands were expected to see waves as high as 11ft, while the Turks and Caicos Islands and south-eastern Bahamas could see towering 20ft waves later in the week, forecasters said.
The National Hurricane Center said Irma was maintaining Category 5 strength with sustained winds near 185mph (295kmh) and heading west-northwest on a path toward Puerto Rico, the Dominican Republic, Haiti and Cuba before possibly heading for Florida over the weekend.
As well as the Leeward Islands, others in the path of the storm include the US and British Virgin Islands and Anguilla, a small, low-lying British island territory of about 15,000 people.
Donald Trump declared emergencies in Florida, Puerto Rico and the US Virgin Islands, and authorities in the Bahamas said they would evacuate the residents of six islands at the southern end of the island chain.
Irma's eye is expected to pass about 50 miles from Puerto Rico later. Hurricane-force winds extended outwards up to 50 miles from the storm's centre and tropical storm-force winds up to 175 miles.