Police are asking for the public's help after a wave of weekend shootings claimed seven lives and left 12 people wounded across Philadelphia.

The violence began about 10 a.m. Friday in the 500 block of York Street, where police answered a call and found James Hall, 58, with a gunshot to the head.

Police arrested Calvin Atkerson, 22, who they said was shooting at another man when the gunfire struck Hall, whom they described as an "innocent bystander who was just sitting there."

About 12:30 a.m., police responding to a report of a home invasion in the 1800 block of South Newkirk Street in the Southbrook Park section found Aaron Tucker, 20, shot multiple times. He was pronounced dead inside the home. Three other men, ages 27, 28, and 42, were also shot in the incident and hospitalized in critical condition.

Police said three men and a woman knocked on the door of the home and, when they were let in, brandished guns and demanded money. When they did not get money quickly enough, they began firing. The four fled on foot, police said.

On Saturday about 3:30 a.m., police were called to the 3300 block of North 13th Street, where they found Shari Harris, 22, who was shot once in the head and pronounced dead at the scene.

Investigators determined that Harris was killed during an apparent robbery. Police said that a gunman shot her and that when she fell to the ground, the man took something from her pockets and fled on foot.

About 3 a.m. Sunday, police responding to a call found Alfredo Santiago, 22, fatally shot inside a silver Honda Accord at Fifth Street and Girard Avenue. Police said Santiago was driving the car when an unknown man started firing at the vehicle.

About 4 a.m., police answered a call at a home in the 2700 block of Erie Avenue, where they found Kevin Underwood, 22, with multiple gunshot wounds. He was pronounced dead a short time later.

Police said Underwood had been sitting in a chair at the address when an unidentified man walked up to him and shot him in the head at close range and then fled on foot.

In the 1600 block of North Lindenwood Street in West Philadelphia about 7:30 p.m. Sunday, police responding to a call found Christopher Lee, 22, shot multiple times.

Police said Lee had been walking with several men when two unidentified men approached, announced a robbery, and went through the men's pockets. Lee, who attempted to run, was shot several times, police said. He was pronounced dead at the scene.

On Sunday about 8 p.m., police were called to 52d and Master Streets, where they found Alonzo Dennis, 28, shot multiple times. He was pronounced dead a short time later at the Hospital of the University of Pennsylvania, police said.

They Dennis had been walking from a store on 52d when he was approached by a man who announced a robbery. When Dennis began to run, the gunman shot him.

Police ask that anyone with information on the shootings call the Homicide Division at 215-686-3334 or 215-686-3335.

Deputy Commissioner Richard Ross decried the shootings as he appealed for the public's help.

"We can't stress enough that we absolutely need some help," Ross said. "There is no question that people know something about this. We don't care how minute the details are. . . . We need some help."

The homicides spanned much of the city.

"Clearly, we're concerned any time you get this amount of gunplay on the streets," Ross said. "Clearly, it could be a kid, it could be a grandmother, it could be an innocent victim. This is always of grave concern to us. I do not believe that the vast majority of people out there are in harm's way."

Contact staff writer Vernon Clark at 215-854-5717