Gunfire erupted under the El at 52nd and Market streets yesterday, sending hundreds of people scurrying in terror as the sidewalks filled with splattered blood and shards of glass.

When the smoke cleared on this humid spring afternoon, a 21-year-old man was dead, his alleged killer was in critical condition and two other men were wounded.

Police said the drama began shortly before 4 p.m. as a young man leaned against the side wall of Valu-Plus on the southwest corner of the intersection.

Video cameras affixed to the side of the store recorded what appeared to be a casual conversation between the young man and two other men who approached him and then walked off.

Without warning, one of the two men stopped and began blasting away at the unidentified man leaning against the store, who collapsed lifelessly on the ground in a pool of blood, police said. He was the city's 141st homicide victim of the year.

A fourth man who was hiding under the stairwell at the bottom of the hulking Market-Frankford El also opened fire, critically wounding the first shooter, police said.

When cops arrived, they found the victims, two revolvers and spent shell casings from a .25-caliber gun that has not yet been recovered. Police said one of the revolvers belonged to the murder victim.

The dead man, his 21-year-old unidentified shooter and a third unidentified 21-year-old man, who was wounded in the foot, were all taken to the Hospital of the University of Pennsylvania. Police said a fourth man wounded in the shootout took himself to Mercy Hospital of Philadelphia with a leg wound.

At one point, a middle-aged man rushed through the crowd towards police. "Oh, no! Where's my son - please!" he yelled. He broke into sobs as he plopped in the back of a patrol car.

"I'm getting ready to get out of Philly and move to Jersey," said David Jones, a street merchant who started selling Mother's Day gifts on the opposite side of Market Street when he noticed the commotion.

"It's getting real bad out here. This corner is as hot as a firecracker," said Jones, 46. The pink balloons and blue teddy bears he sold for $25 didn't draw many customers.

"Nobody wants to stop," he sighed. "Everyone wants to keep moving before something else happens." *