(CNN) -- Much of downtown Nashville remained under a blanket of murky water after a massive system of rain and thunderstorms pounded much of Tennessee over the weekend.
The severe weather was blamed for at least 27 deaths -- 18 in Tennessee -- across the Southeast between Saturday and Monday, emergency officials said. Ten of those deaths occurred in Davidson County including Nashville, the Nashville mayor's office said.
Nashville residents said that despite forecasts of rain going into the weekend, they had little time to prepare for the floodwaters that crept into homes, washed away roads, prompted evacuations of hotels and displaced thousands of people.
"We all knew that there was going to be heavy rains this weekend, but Nashville normally gets about 5 inches of rain in the month of May and nobody could have predicted 15-16 inches of rain in 48 hours," resident John Rives said.
CNN iReporter Tom Frundle said the flooding is as bad as he's ever seen.
"I've lived in different parts of the country, and I've never seen anything like this," he said, a sentiment echoed by many Tennesseans, including Gov. Phil Bredesen.
"It's an astonishing sight for someone who has lived here a long time," the governor said.
Bredesen, who toured the affected region by helicopter Monday with Nashville Mayor Karl Dean, told a news conference "this is obviously a major, major event."
More than 13 inches of rain fell at Nashville's airport, Bredesen said, with higher totals elsewhere in the region.
Federal Emergency Management Administrator Craig Fugate was on hand Monday to survey the damage and offer FEMA's support in the state's recovery efforts.
"This is going to be a long time in cleaning up," Bredesen said in asking Fugate to sign disaster declaration papers.
Though the sun re-emerged Monday, the floodwaters showed no sign of receding as the Cumberland River, which runs through downtown Nashville, continued to rise. The river was expected to crest at 52.5 feet by 8 p.m. Monday, more than 12 feet above flood level.
The water wasn't expected to recede until late Tuesday night, according to the National Weather Service.
All eyes were on Nashville's iconic music institutions, many of which are located in hard-hit downtown. Water flooded the lower levels of the Country Music Hall of Fame, the Grand Ole Opry and the Schermerhorn Symphony Center, according to The Tennessean newspaper.
The Grand Ole Opry announced Monday on its website that it will move performances this week to other venues unaffected by the flooding, marking the first time since 1975 the concert hall has been forced to make such a move. The 1975 closing was also caused by Cumberland River flooding.
"While we ourselves are shaken by the impact of the flooding of the Opry House and throughout the area, it is important that Nashville's most treasured tradition continues with this week's shows," Grand Ole Opry Vice President Pete Fisher said in a statement. "We look forward to coming together both as the Opry family and as a great American city just as we have every week for nearly 85 years. Our hearts go out to all of those affected in the Middle Tennessee area."
Opry officials said it is too early to know how long operations will be affected.
Rives said most residents in his neighborhood -- where he estimated about a dozen homes were submerged -- were staying put in their homes, avoiding the maze of road closures.
Some of those who have needed to venture out were ferried by their neighbors by boat across flooded roads, Rives said.
At the Opryland hotel, a luxury resort inside the complex that includes the Grand Ole Opry, guests were evacuated by shuttle bus Sunday to spend the night on cots at McGavock High School in Nashville.
Kim Keelor, a spokeswoman for the resort, said two of the hotel's three atriums "have become lakes."
"Nine acres of beautiful gardens, priceless plants, restaurants and shops -- all under water," she said Monday, adding, "it's kind of surreal because it's a beautiful day."
U.S. Rep. Jim Cooper, D-Tennessee, called the damage to the state devastating.
"We don't know how long the cleanup will take," he said. "My estimate would be this is a multibillion dollar disaster."
About 22,000 homes were still without power Monday across central Tennessee, the state's emergency management office said, and residents in Davidson and Williamson counties were being asked to cut their water use by 50 percent.
Nashville's K.R. Harrington Water Treatment Plant was under water and expected to remain closed for several days, and the city's other treatment plant was under threat from the rising river, officials said.
"It is vitally important in the days ahead as the water supply gets tighter, we do everything we can to stretch that out," the mayor said.
Elsewhere in the Southeast, residents in Mississippi and Kentucky were also reeling from powerful storms.
Six people were killed in northern Mississippi, where a storm spawned tornadoes, flooded roads, downed power lines and damaged homes.
Three people died in storm-related incidents in southern and south central Kentucky, an emergency services spokesman said Monday.