A weeklong survey of the city's homeless found 528 people living on the streets, slightly more than half of them described as "physically vulnerable and at increased risk of death."
The count was conducted by 250 volunteers, who combed city streets and parks this week from 4 to 6 a.m. compiling a name-and-photo database.
Mayor Nutter hosted the volunteers Friday at City Hall and helped release the results of the survey.
"The big-picture goal remains the same," he said. "Philadelphia will become the first major city in America to end homelessness."
The count was part of a national campaign to house, by July 2013, 100,000 people now living on the streets.
Spearheaded by a New York City nonprofit called Common Ground, the initiative grew out of the group's efforts to help house homeless people living in Times Square.
Jake Maguire, a spokesman for the 100,000 Homes Campaign, said Common Ground began intensive outreach in 2003, identifying the most vulnerable people on the streets and trying to place them first in permanent housing.
Other communities across the country began seeking out Common Ground for advice on how to help the chronically homeless. In July, the nonprofit launched a national effort to help other cities with outreach and placement efforts, Maguire said. Common Ground has created a "vulnerability index" to identify individuals who need help the most.
The index uses a 34-item questionnaire that gathers information about health, institutional history, length of homelessness, and other data.
In the Philadelphia survey, 268 homeless people qualified as vulnerable, 44 of whom were veterans. In the total homeless population, 15 people are older than 65, and 189 reported being the victim of a violent crime since becoming homeless.
Nutter also said that, in the last two years, 90 homeless people in the city have died.
Currently, 83 communities have signed on to participate in the effort, with about 8,000 individuals housed so far, Maguire said. Nutter said he hoped housing could be found in the next six months for 50 of those people identified this week.
The city, working with an established network of nonprofits serving the homeless, has long supported outreach efforts. Every day, experts with social service agencies fan out across the city trying to reach out to people and persuade them to come off the streets.
In addition, the city conducts a quarterly count of the city's street population, focusing on Center City and other key locations. The most recent census, for the winter of 2011, showed 359 people living on the streets.
The connection between mental illness and street homelessness has been well-established by local groups that work with homeless populations.
About 85 percent of the people existing on Philadelphia's streets suffer from mental illness, addictions, or both, according to the city's mental health offices.