At least seven people were rescued from the Saint Sophia Camp in Waterman Canyon, just north of San Bernardino, and needed medical attention, authorities said. Details about the victims, the extent of their injuries and the status of the other eight people were not immediately available.
A sheriff's department spokeswoman said there were reports of as many as 18 others still trapped.
Television reports showed a surging stream of water in the canyon, which was a sea of gray mud.
The mudslide occurred exactly two months after a wildfire started in the upper portion of the canyon in the San Bernardino Mountains. The fire eventually consumed just over 91,000 acres.
Wildfires make Southern California's mountains much more prone to mudslides because they burn off vegetation that normally would help shore up steep terrain. The fires that hit the region in October and November were the most severe in state history, burning nearly 1 million acres.
Flood waters in Waterman Canyon were getting worse late Thursday, forcing officials to pull back some emergency personnel, county fire officials said. Authorities evacuated residents of Waterman Canyon and closed off the road leading to the canyon.
A bridge washed out in the canyon and several structures were threatened, said Clifford Ellis, a supervisor at the San Bernardino County Fire Department.
Sheriff's spokeswoman Cindy Beavers said search and rescue crews ``are having to go around the mountain and come down into the area where the mudslide has occurred, which is only delaying the rescue operation.''
The camp hosts religious retreats for children ages 7 to 17 during the summer months as well as other events year-round, according to its Web site. No one answered the phone at the camp on Thursday. Messages left with camp officials were not immediately returned.
Elsewhere in San Bernardino County, east of Los Angeles, authorities ordered residents to evacuate as mudslides threatened homes. Several roads were closed.
In Lytle Creek Canyon, the rain caused several mudslides, including a 4-foot-high flow across a road that trapped a car. The driver was not hurt and the road was closed.
Emergency officials ordered an undetermined number of residents along the overflowing creek to evacuate.
Strong wind gusts also downed power lines and disrupted service to various areas of Los Angeles, authorities said. Hundreds of people were without power.
The storm began moving into wildfire-scarred Southern California Wednesday evening, bringing the first rainy Christmas Day in Los Angeles in 20 years. The National Weather Service issued a flash flood warning for southwestern San Bernardino County, including areas around Lytle Creek.
In a 24-hour period beginning Wednesday afternoon, 3.57 inches of rain fell in Lytle Creek, said Stan Wasowski, a National Weather Service forecaster.
``It'll probably get worse before it gets better.'' he said.
Minor flooding also was reported on some roads in Crestline and in residential areas of Devore, a suburb of San Bernardino.
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