By Steve Gorman

(Reuters) -Torrential rains unleashed flash floods along the Guadalupe River in Texas on Friday, killing at least 24 people as rescue teams scrambled to save dozens of victims trapped by high water or reported missing in the disaster, local officials said.

Among the missing were 23 to 25 people listed as unaccounted for at an all-girls Christian summer camp located on the banks of the rain-engorged Guadalupe, authorities said.

The U.S. National Weather Service declared a flash flood emergency for parts of Kerr County in south-central Texas Hill Country, about 65 miles (105 km) northwest of San Antonio, following thunderstorms that dumped as much as a foot of rain.

Dalton Rice, city manager for Kerrville, the county seat, told reporters the extreme flooding struck before dawn with little or no warning, precluding authorities from issuing advance evacuation orders as the Guadalupe swiftly rose above major flood stage.

"This happened very quickly, over a very short period of time that could not be predicted, even with radar," Rice said. "This happened within less than a two-hour span."

State emergency management officials had warned as early as Thursday that west and central Texas faced heavy rains and flash flood threats "over the next couple days," citing National Weather Service forecasts ahead of the holiday weekend.

But the weather forecasts in question "did not predict the amount of rain that we saw," W. Nim Kidd, director of the Texas Division of Emergency Management, told a news conference on Friday night.

July Fourth fireworks displays ended up being canceled in flood-stricken communities throughout the region, including Kerrville, where the waterfront site for Friday night's planned U.S. Independence Day celebration was submerged by the rain-swollen river.

At Friday night's briefing, Kerr County Sheriff Larry Leitha said 24 flood-related fatalities had been confirmed, up from 13 tallied earlier in the day.

One more person found dead in neighboring Kendall County was not confirmed to be a flood-related casualty, Leitha said.

PRAYERS FOR THE MISSING

Lieutenant Governor Dan Patrick said on Friday afternoon that authorities were searching for 23 girls listed as missing from among more than 750 children at summer camp sites along the banks of the Guadalupe River when the area was inundated by floodwaters at around 4 a.m. local time.

The missing campers had all been attending Camp Mystic, a private Christian summer camp for girls.

"We're praying for all those missing to be found alive," Patrick said.

It was not clear whether anyone unaccounted for might have ended up among the deceased victims tallied countywide by the sheriff.

Otherwise, all other campers were safe, authorities said, with campers being evacuated throughout the day, officials said.

"Everybody is doing everything in their power to get these kids out," Kerr County Judge Rob Kelly, the top local elected official, had said at a news briefing on the disaster hours earlier.

Kelly said a number of scattered residential subdivisions, recreational vehicle parks and campgrounds were hit hard.

Pressed by reporters why more precautions were not taken with stormy weather in the forecast, Kelly insisted a disaster of such magnitude was unforeseen.

"We have floods all the time. This is the most dangerous river valley in the United States," Kelly said. "We had no reason to believe this was going to be anything like what's happened here. None whatsoever."

In an alert on Thursday, the Texas Division of Emergency Management said it had increased its readiness level and "activated additional state emergency response resources" as parts of west and central Texas braced "for continued heavy rainfall and flash flooding threats heading into the holiday weekend."

Lieutenant Governor Patrick said the Guadalupe River had risen 26 feet (8 m) in 45 minutes as heavy showers soaking the region.

As of Friday night, emergency personnel had rescued or evacuated 237 people, including 167 by helicopter.

With additional rain forecast in the region, Patrick warned that an ongoing threat for possible flash flooding extended from San Antonio to Waco for the next 24 to 48 hours.

On Friday night, Texas Governor Greg Abbott signed a disaster declaration to hasten emergency assistance to Kerr and a cluster of additional counties hardest hit by the floods.

Personnel from the U.S. Coast Guard and the Federal Emergency Management Agency also were activated to assist local authorities in confronting the crisis, officials said.

(Writing and reporting by Steve Gorman in Los Angeles; Additional reporting by Ismail Shakil in Ottawa, Jasper Ward in Washington, Acharya Bhargav in Toronto and Daniel Trotta in Carlsbad, California; Editing by Matthew Lewis and Tom Hogue)