The vigil was organized by Heeding God's Call, which executive director Bryan Miller described as a "faith-based group movement to prevent gun violence."
The group targets gun shops it says sell firearms to "straw buyers," who then sell them to criminals on the streets.
Few of those at the vigil, however, appeared to be residents of the tidy, working-class neighborhood adjacent to Haverford and Lower Merion in which drug-related violence is extremely rare, Philadelphia police officials say.
Some residents did watch through screen doors as television news crews arrived, but they did not come out.
The lawn of the house where the killing occurred still had the makeshift shrine of stuffed animals left there when the teens were killed.
Before the vigil got under way, a man who said he was a friend of the teens' parents - Rohan and Cynthia Bennett - placed black trash bags of the family's possessions near the shrine.
The Bennetts were arrested after the slayings on drug-related charges, police said.
The Rev. Dolores McCabe, retired pastor of Mill Creek Baptist Church, said her daughter once had Christopher Malcolm in her class.
"We come here to commemorate the fact that two lives have been lost," she said. "We are sorrowful for two little boys who are old enough to be my grandchildren. We hope what we do here today provides comfort for people, but we also need to speak to the neighborhood that this violence needs to end."
McCabe asked for prayers not only for the victims and their families, but for the "perpetrators, that they no longer inflict pain on others."
The Rev. Frederick A. Hanna, pastor of the First African Presbyterian Church on West Girard Avenue, told the group he was "challenging you as I challenged members of my church this morning not to be silent about gun violence because we can make a difference."
In the litany he read, Hanna referred to those killed and wounded July 20 when a gunman opened fire on a crowd of moviegoers in an Aurora, Colo., theater.
McCabe noted the reference to Aurora.
"This litany was written for a vigil on July 15, and we had not yet rewritten it," she said. "Consider that it was only July 15, and already more have died."
Miller said his group's efforts were not against gun shops that followed the law and sold firearms legally.
"It is the gun shops that are selling to traffickers," he said. "There is much that we can do to stop this."
Contact Alan J. Heavens at 215-854-2472, firstname.lastname@example.org, or follow @alheavens at Twitter.