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HOUSTON — Rescuers answered thousands of calls from people trapped in Houston on Sunday as torrential rain from deadly Hurricane Harvey caused "catastrophic flooding" in the city and across southeast Texas, officials said.

"This event is unprecedented & all impacts are unknown & beyond anything experienced," the National Weather Service said Sunday morning.

Heavy rain continued to pour over Houston early Sunday afternoon, with some downtown areas knee-deep in water, and shut down portions of highways flooded with as much as 10 feet of water.

At least one person has died in flooding in Houston, officials said. The storm is blamed in at least two deaths overall.

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Hurricane Harvey, currently a tropical storm, has caused "catastrophic and life-threatening flooding" in southeast Texas, according to the National Weather Service (NWS)

At least two deaths have been blamed on the storm.

Houston officials said rescuers have fielded more than 2,000 calls for help

Harvey is expected to bring 15 to 25 inches of additional rainfall to the middle and upper Texas coast through Friday, according to the NWS.

Houston police said Sunday afternoon that more than 1,200 people had been rescued, with more to come. The weather service warned that flooding victims should go to their rooftops and not their attics to avoid being trapped by the rushing waters. Police in the city of Dickinson, southeast of Houston, made an online plea for people with boats to help in rescues.

"This is a life-threatening situation," said Michael Palmer, lead meteorologist at The Weather Channel.

In parts of south Houston, there were multiple abandoned cars and manhole covers spewed rushing water. As the rain slowed Sunday afternoon, one car blocked a roadway where it had stalled when the flood waters were high.

Some parts of Houston and just west of the city could receive a Texas record of 50 inches of rain as the tropical storm continues to stall over the area, NWS meteorologist Patrick Burke told the Associated Press.

The NWS said local rainfall of 50 inches "would exceed any previous Texas rainfall record."

"The breadth and intensity of this rainfall are beyond anything experienced before," the agency's Weather Prediction Center said in a tweet.

Earlier Sunday, the National Weather Service said that the Houston and Galveston area had received 24.10 inches of rain in the last 24 hours.

GALLERY: Hurricane Harvey: Life-threatening Rain Hammers Houston

The average rain fall within the Harris County emergency management network had already exceeded that of Tropical Storm Allison in 2001 in almost half the time, the Weather Prediction Center said.

The National Hurricane Center said that Allison caused 41 deaths, with 23 in the state of Texas alone, and an estimated $5 billion in damages.

President Donald Trump will be traveling to Texas on Tuesday after coordinating with state and local officials, said White House Press Secretary Sarah Huckabee Sanders.

"We continue to keep all of those affected in our thoughts and prayers," she said.

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Texas Gov. Greg Abbott said Sunday afternoon that 3,000 national and state Guard personnel have been activated in the state.

"One thing that we are dealing with here is basically a triangle of ongoing rain that stretches one tip of which would be the Corpus Christi area, up to Travis county, over to Chamber's county and then back to Corpus Christi," he said during a news conference. "Parts of those regions will continue to receive incredibly heavy rain that will lead to even more flooding and more danger for Texans."

Abbott stressed "the importance of staying off the road" during the heavy rains and urged people to avoid going outside if possible.

"If you drive in the water, you're taking your life in your own hands," he said.

University of Houston evacuated its fraternities and sororities. Some students had to leave behind their vehicles #HoustonFlood2017 pic.twitter.com/bEY2WIQYpN — Phil McCausland (@PhilMcCausland) August 27, 2017

University of Houston students Kevin Ford and Jimmy Taravella, both 20, surveyed the flooding outside the college's fraternity housing on Sunday afternoon and said the storm was worse than they imagined.

"This is worse than I thought it was going to be," said Taravella

"This is the first time I’m seeing it and it’s pretty bad," said Ford.

Ford said university officials said some fraternity and sorority buildings had to be evacuated. Some students had to leave behind cars that were stuck in water several feet deep and as high as their engines. In several low-lying areas on campus, multiple vehicles were engulfed in water.

Driving through heavy rain on our way to Houston for @NBCNightlyNews tonight. Floods have cutoff many access routes to city. pic.twitter.com/29ixnwC3Bs — Lester Holt (@LesterHoltNBC) August 27, 2017

There was a heavy police presence in the city on Sunday as officials continued to rescue people from the heavy flooding. At the university, campus police continuously patrolled the area, encouraging curious students to head back inland.

As responders rushed to save people from flooded homes and stranded cars, Houston city officials said emergency services were "at capacity" and warned residents to "shelter in place" and not to call 911 unless they were in "imminent danger."

Houston officials have received more than 2,000 calls for rescue since the storm made landfall, Mayor Sylvester Turner said at a press conference Sunday.

"We have had an unprecedented amount of water," Turner said. "I don’t think I need to tell anyone at this point that this is a very, very serious and unprecedented storm," he said.

Kayakers try to beat the current down an overflowing Brays Bayou in Houston on Sunday. Mark Mulligan / Houston Chronicle via AP

Houston had opened up a city convention center as a shelter for people impacted by severe flooding, he added, but encouraged people to stay home if they could.

"The safest place is for you to be in your home," he said.

The south side of the city was being deluged by up to six inches of rain every hour overnight, the Office of Emergency Management said.

The U.S. Coast Guard said it was conducting urban search and rescue operations in the greater Houston area and that its local command center had received more than 300 requests.

"Currently there are five MH-65 Dolphin Helicopters conducting rescues," the statement said.

The flooding came as emergency services along the Gulf Coast scrambled to reach those in need of assistance, and many in the path of the storm began to assess the devastation.

Brock Long, administrator of the Federal Emergency Management Agency (FEMA), said on NBC News' "Meet The Press" Sunday morning that the recovery effort would take "years."

Long told host Chuck Todd that President Donald Trump was "extremely concerned" about the situation on the ground.

"He’s given me all the authorities to amass the resources from the federal government down through our state and local partners," he said adding, "right now we have nearly 5,000 staff that we have coordinated across the federal government within the states of Texas and Louisiana."

At least two people have died since Harvey made landfall late Friday as a Category 4 hurricane. One was found dead in a vehicle in Houston on Saturday night in a possible drowning amid flooding in the city, a Houston fire department spokesperson said. Another died in a house fire in Aransas County that rescuers could not reach because of flooding.

Hurricane Harvey batters downtown Houston on Saturday. Scott Olson / Getty Images

The storm was about 15 miles northeast of Victoria as of 8 p.m. ET Sunday with 40-mph winds, and it is causing "unprecedented flooding," the National Hurricane Center said.

"When you get rates of over six inches of rainfall an hour it overflows the drainage systems and homes and businesses will be flooded, which is what we're seeing now in Houston," Palmer said.

A tornado watch has been issued for parts of Louisiana and Texas until 1 PM CDT pic.twitter.com/7bgyWZWtUE — NWS Houston (@NWSHouston) August 27, 2017

He warned that the storm was likely to stay in the same area for the next few days meaning rainfall would continue to fall over southeast Texas. "The flooding is only going to get worse as we go forward," he said.

Current forecasts see total rainfall between Sunday and Thursday reaching 40-50 inches, which "would be one of the worst floods in U.S. history," Palmer said.

More than 80,000 people were without power in the Houston area, CenterPoint Energy said. City officials warned residents to stay at home as rescue operations were carried out.

Full Coverage: The Latest on Tropical Storm Harvey

A pregnant woman going into labor was among the rescued, Harris County Sheriff Ed Gonsalez said early Sunday.

"There are a number of people on our streets calling 911 exhausting needed resources. You can help by staying off the streets," said Turner, Houston's mayor.

Houston Police Chief Art Acevedo posted a warning on Twitter, saying: "Cannot emphasize enough how much flooding there is on roadways you are endangering yourself and our first responders by being out stay put ... do not think it's safe to be driving anywhere in the city."

Empty city buses line up on Interstate 59 near Houston on Saturday in case their bus shelters flood. Mark Mulligan / AP

He added: "Have reports of people getting into attic to escape floodwater do not to do so unless you have an axe or means to break through onto your roof."

Houston’s William P. Hobby Airport and Houston Bush Airport said all commercial flights are canceled until further notice.

Trump signed a disaster declaration late Friday, allowing federal funding to help stricken areas.

Trump said on Twitter Sunday: "Many people are now saying that this is the worst storm/hurricane they have ever seen. Good news is we have great talent on the ground."

Phil McCausland reported from Houston, Daniella Silva reported from New York and Saphora Smith reported from London.