Puerto Rico Governor Ricardo Rossello called Maria 'the most devastating storm in a century.'
San Juan: Hurricane Maria devastated Puerto Rico on Wednesday and left the entire island without power, bringing deadly winds and pounding rain that sent thousands scurrying to shelters.
Maria slammed into Puerto Rico's southeast coast at daybreak before churning across the US territory which is home to 3.4 million people.
The storm was blamed for 10 deaths in the Caribbean, including a man in northern Puerto Rico's Bayamon who died after being struck by a board he had used to cover his windows, government spokeswoman Yennifer Alvarez said.
Though the storm had moved back out to sea, authorities early Thursday declared a flash flood warning for all of Puerto Rico.
"If possible, move to higher ground NOW!" the National Weather Service station in San Juan said in a tweet, calling the flooding "catastrophic."
Puerto Rico Governor Ricardo Rossello called Maria "the most devastating storm in a century."
"We have a lot of flooding, a lot of infrastructure damage, telecommunication system is partially down, energy infrastructure is completely down," he told CNN.
Rossello added that authorities did not have much information from the island's southeast, which was "virtually disconnected" after taking a direct hit from Maria when it made landfall.
Tens of thousands of people had hunkered down in shelters in the capital San Juan as the storm approached. Mayor Carmen Yulin Cruz broke down in tears as she spoke of the utter devastation she had witnessed.
"Many parts of San Juan are completely flooded," Yulin Cruz told reporters in one of the shelters, its roof swaying while she spoke.
"Our life as we know it has changed."
Maria made landfall as a Category Four storm on the five-point Saffir-Simpson scale, initially packing winds of a little over 150 mph (240 kph) before easing slightly as it powered towards San Juan.
"The wind sounds like a woman screaming at the top of her lungs!" Mike Theiss wrote on Twitter, sheltering in a safe room in the eye of the storm.
Imy Rigau, who was riding out the storm in her apartment in San Juan, said water cascaded through her ceiling.
"We are taking refuge in the hallway as there is about a foot (30 centimeters) of water in my apartment," she told AFP.
- 500 shelters -
Many of the most vulnerable of Puerto Rico's residents took cover in the 500 shelters set up around the island.
Rossello imposed a 6:00 pm to 6:00 am curfew until Saturday and warned of flooding and mudslides.
"I urge the people of Puerto Rico to commit to peace, understanding, and good judgment during these difficult times for our island," he said.
As night fell, there were reports of looting and authorities said 10 people had been arrested.
Puerto Rico's most catastrophic hurricane was in 1928 when Hurricane Okeechobee -- also known as San Felipe Segundo -- killed 300 people.
Although engineers had managed to restore power to most of the island after the recent Hurricane Irma, Maria caused a new black-out across the island.
Brock Long, who heads the US federal government's emergency agency FEMA, said it could take days for power to be restored on Puerto Rico and the smaller US Virgin Islands which have also been badly hit.
Rossello's assessment for when the lights might come back on was much more grim.
"It depends on the damage to the infrastructure," he told CNN. "I'm afraid it's probably going to be severe. If it is... we're looking at months as opposed to weeks or days."
- Dominica devastation -
The US and British Virgin Islands -- still struggling to recover from the devastation of Irma -- are also on alert, along with the Turks and Caicos Islands and parts of the Dominican Republic.
On Wednesday evening, the hurricane was about 75 miles (120 kilometers) east of Punta Cana in the Dominican Republic and had been downgraded to a Category Two storm, according to the National Hurricane Center.
Maria has already torn through several Caribbean islands, leaving at least seven people dead on Dominica.
Communications to Dominica have been largely cut, and its airports and ports have been closed.
But an advisor to Prime Minister Roosevelt Skerrit, who spoke to the premier by satellite phone, painted a picture of devastation on the island, where around 73,000 people live.
"It's difficult to determine the level of fatalities but so far seven are confirmed, as a direct result of the hurricane," Hartley Henry said in a statement.
Reports from rural communities spoke of a "total destruction of homes, some roadways and crops," added Henry.
"The country is in a daze -- no electricity, no running water -- as a result of uprooted pipes in most communities and definitely no landline or cellphone services on island, and that will be for quite a while."
In the French territory of Guadeloupe, one person was killed by a falling tree as Maria hit, while another died on the seafront.
At least two are missing after their boat sank off the French territory, while 40 percent of households were without power.
There were fears that Maria could wreak fresh havoc on islands that were already flattened by Category Five Hurricane Irma earlier in the month.
Reports suggested St Martin, a French-Dutch island that was among the most severely hit by Irma with 14 dead, had escaped the worst this time around.