Police received a report of a fight in front of the Walmart at 15555 Hesperian Blvd. around 1:50 a.m.
Officers already patrolling near the parking lot went to the center lot aisle near the front of the store where they found a victim suffering from a gunshot wound and the victim's family members detaining a possible suspect, police said.
It appears the victims were walking to their car when multiple suspects confronted them, police said.
After suspects asked the victims for their items and were refused, a fight ensued. One suspect pulled a gun and shot one of the victims.
The victim was taken to a hospital in critical, but stable condition, police said.
Family members wrestled down one of the suspects as another fled.
Once officers arrived, the detained suspect -- a man in his mid-20s -- was arrested, police said.
It is unknown if the arrested man was the gunman in the incident.
The Hesperian Boulevard Walmart listed its doors opening Thursday night at 10 p.m. for early Black Friday shopping and deals.
San Leandro police Sgt. Mike Sobek told reporters at the scene this morning it was a "robbery gone bad."
Walmart parking lot entrances were reportedly blocked by numerous police cars at 4 a.m. after the shooting, a clerk said from the Big 5 Sporting Goods across the street.
Police said investigators are still at the scene this morning.