The impact hurled Merke from his bike. The driver of the car remained at the scene and the crash is still under investigation, police said. Milan was wearing a helmet, police said.
Merke was placed in a medically-induced coma to relieve pressure and bleeding on his brain, said his friends and fellow riders who remained gathered at Hahnemann University Hospital until late into night Friday.
Saturday, about a dozen of Merke's fellow motorcycle club members held vigil outside the hospital -- their bikes lined up in front of the emergency entrance.
For a time, Merke's family members and fellow officers came outside to pray with the bikers.
A member of the Philly X-Treme Motorcycle Club, Merke was planning to join 100 other riders in the Midnight Run for Peace through violence-torn neighborhoods.
"We go through high-crime areas and try to talk to the community," said Richard Dukes, of Men United for a Better Philadelphia, who was at the hospital Saturday.
"He's been with us from the inception," Dukes said of Merke. "He's a good person - a wonderful person to be around. He wants to give back to the community."
Merke had regained consciousness Saturday and was improving, Dukes and others said
"We're out here to support him and his family," Dukes said. "He's part of our family too."
Contact staff writer Julie Zauzmer at 215-854-2771 or firstname.lastname@example.org