LOS ANGELES, OCT. 31 -- River Phoenix, who emerged as a young actor in the coming-of-age film "Stand by Me" and costarred in the offbeat "My Own Private Idaho," died shortly after collapsing outside a Sunset Strip nightclub early this morning. He was 23.

Phoenix had been "acting strange" before leaving the Viper Room, investigators were told by his brother, Joaquin Phoenix, 19, and actress Samantha Mathis, 23, who had accompanied him to the club, opened recently by actor Johnny Depp. Witnesses said he appeared to have gone into some kind of seizure before being taken outside, KNBC television reported.

After he collapsed at about 1 a.m., paramedics rushed Phoenix to Cedars-Sinai Medical Center. He was pronounced dead at 1:51 a.m., according to the Los Angeles County Sheriff's Department. An autopsy report determining the cause of death is expected Monday.

The death is being investigated by the Sheriff's Department's Homicide Bureau, which is routine. "This is not being handled as a homicide," said Deputy Diane Hecht. "In this jurisdiction, the homicide bureau investigates any unexplained deaths."

Unlike many child stars who fell into drug abuse and alcoholism under Hollywood's bright lights, Phoenix had maintained a largely clean-cut reputation. He was a strict vegetarian who often spoke out publicly against eating meat.

Depp's Viper Room is popular with young celebrities, providing a stage recently for a listening party for Concrete Blonde and an impromptu tuneup show by Pearl Jam. Depp, the heartthrob who starred in the Fox television series "21 Jump Street" and the movies "Edward Scissorhands" and "Benny & Joon," took over what had been Club Central, a trendy dive.

"It comes as a total shock that this extraordinarily talented young man's life would end so abruptly. It's just tragic," said his publicist, Sue Patricola. She said Phoenix's family was gathering in Los Angeles.

Phoenix was in the middle of filming the movie "Dark Blood," a Fine Line Features film that was nearly complete, Patricola said. He was also to have costarred in the film version of Anne Rice's best-selling novel "Interview With a Vampire" with Tom Cruise, which was scheduled to begin filming this month.

Known for his natural style and dramatic intensity, Phoenix made his film debut in 1985 in "Explorers." He gained fame as a scruffy, cigarette-smoking youth, Chris Chambers, in Rob Reiner's 1986 hit "Stand by Me," the story of a group of boys who discover a dead body.

Phoenix won critical praise for his performance in Gus Van Sant's

"My Own Private Idaho," a 1991 road movie in which he played a wandering male street prostitute suffering from narcolepsy. His other film credits include "Indiana Jones and the Last Crusade" and "The Mosquito Coast" (both starring Harrison Ford), "A Night in the Life of Jimmy Reardon," "Little Nikita" (with Sidney Poitier) and "Running on Empty." In 1992, he played a youthful computer hacker in the Robert Redford-led ensemble cast of "Sneakers."

Born in Madras, Ore., on Aug. 23, 1970, Phoenix spent most of his childhood in Venezuela with his siblings, Rainbow, Leaf, Liberty and Summer, while his parents, John and Arlynn Phoenix, worked as missionaries for Children of God, a counterculture religious sect. He was named after the river of life in Herman Hesse's novel "Siddartha." Phoenix once summed up his unique origins in an interview:

"I was born in Oregon. My sister, Rain, was born in Texas. My brother, Joaquin Rafael, alias Leaf, was born in Puerto Rico. My other sister, Liberty, is a native of Venezuela. My other sister, Summer, was born in Florida, my mother in the Bronx, N.Y., my father in Fontana, Calif."

Family legend has it that River began his performing career at age 5, singing with 3-year-old Rain on street corners in Caracas. He saw his nonconformist early life as an asset to his career as an actor.

"I think it helps you become versatile in adapting to new situations, new atmospheres, new people, just as you do when you go from film to film," he said five years ago.

The family eventually moved to Los Angeles when Phoenix was 10. He appeared in commercials before he was cast in the 1982 television series "Seven Brides for Seven Brothers."

Special correspondent Martin Booe contributed to this report.