The storm has been downgraded to a tropical storm as it weakens over land, but heavy rains and widespread flooding are likely to continue. More than 200,000 people are without electricity as reports of damage trickle in.
The storm has ripped off roofs, snapped powerlines, and triggered tornadoes and flash floods.
Hurricane Harvey, the most powerful storm to hit the US state of Texas since 1961, made landfall on Friday evening, as authorities warned of catastrophic ocean surges and winds.
Texas Governor Greg Abbott said he would activate 1,800 members of the military to help with the statewide clean-up while 1,000 people would conduct search-and-rescue operations.
Officials and media in the coastal city of Rockport, where the first fatality has been reported, are already reporting significant damage.
They said that a high school, a hotel, a senior housing complex and other buildings being used as shelters had been damaged by the storm.
Rockport's acting mayor earlier had advised people to write their name and social security number on their arms in permanent marker.
"We hate to talk about things like that. It's not something we like to do but it’s the reality. People don’t listen," Patrick Rios told reporters Friday, according to media reports.
At a recreational vehicle sales lot, a dozen vehicles were flipped over and one had been blown into the middle of the street.
"It was terrible," said resident Joel Valdez. The storm ripped part of the roof from his trailer home at
around 4 a.m., he said. "I could feel the whole house move."
Valdez, 57, said he stayed through the storm to look after his animals. "I have these miniature donkeys and I don't know where they are," he said, as he sat in a Jeep with windows smashed by the storm.
Hundreds of thousands of people on the coast were without power early on Saturday morning, according to the local Houston Chronicle.
Cities from northern Mexico to Louisiana were bracing for flooding.
Tens of thousands of residents fled the "life-threatening" winds and rains set to slam the coast and potentially swamp counties as far as 100 miles (161 kilometers) inland. As many as 5.8 million people were in the forecast path of the storm path. Governor Greg Abbott had earlier warned coastal residents to get out of harm's way immediately.
US President Donald Trump signed a disaster proclamation for the state as Hurricane Harvey strengthened to Category 4.
After the storm hit the Texas coast with 209 kph (130 mph) winds, it was later downgraded to a Category 1 as it settled over the southeast of the state.
The storm is expected to slow further and flood the area with rain up to the middle of next week.
Governor Abbott urged residents of Houston, a city of 2.3 million people, to head north away from the coast. But the county's top elected official insisted that residents of the city should not evacuate. Ed Emmett, the Harris County judge, said although the hurricane was expected to dump lots of rain on Houston, the city wasn't expected to receive the large amounts of storm surge expected in other areas. Houston Mayor Sylvester Turner urged residents to stay in their homes and off the roads.
Federal Emergency Management Agency (FEMA) chief Brock Long had earlier warned eastern Texans that the window to be evacuated was closing rapidly. "Texas is about to have a very significant disaster," Long said.
Life-threatening storm surge
Forecasters predict that the hurricane has the potential to douse the coast in 12 feet (4 meters) of water.
Late Friday forecasters warned that the storm was "spreading onto the middle Texas coast."
"Catastrophic and life-threatening flooding is expected across the middle and upper Texas coast," the National Weather Service warned.
Astronauts aboard the international space station shared images of the weather system from space.
'Good luck' Trump says
Trump wished "good luck to everybody" in the path of the storm, as aides distributed photos of him being briefed in the Oval Office.
The US president said on Twitter that the federal government was on site and ready to respond.
Trump is expected to travel to the area early next week, said White House Press Secretary Sarah Huckabee Sanders. After the disastrous reaction to Hurricane Katrina by then-President George W. Bush in 2005, many in the United States are watching how the Trump administration will handle what may be its first major natural disaster.
Cruise passengers stuck
About 20,000 passengers on three cruise ships were stuck after the Galveston port closed. Two were rerouted to New Orleans while a third was to remain in Mexico.
Petrol stations were running out of fuel as thousands of residents fled the region. Meanwhile, prices spiked as oil refineries shut down.
tj,bik/jlw (AP, Reuters)