Addressing a crowd at Girard College before the workday began, Vice President Joe Biden said that King taught that every person, regardless of background, education or capacity, had something to give.
"Every single person can be great, because every single person can serve," Biden said, noting that before legislation creating MLK Day was passed 26 years ago, opponents had complained that the holiday would simply be a drain on taxpayers.
Instead, Biden said, it has become a day of great generosity.
"The greatest thing you can do for another human being, in my opinion, is to let them know you understand what they're going through and then try, try, just engaging in one act of kindness," he said.
Mayor Nutter called Philadelphia "a city of service," noting that this year's event had drawn a record number of volunteers - 4,000 at signature site Girard College alone - participating in 1,300 projects throughout the region.
When the first day of service was held in 1996, about 1,000 people in the region took part.
"Let's take this energy and passion in this room today and make service a part of our lives every day," urged Todd Bernstein, founder and director of the Greater Philadelphia King Day of Service.
Biden and Nutter joined the food-relief organization Stop Hunger Now in putting together more than 60,000 care packages for people in need around the globe. They joined dozens of hairnet-clad volunteers at Girard College who carefully measured out rice, vegetables and protein, added vitamin packs and then sealed the completed meals.
"When you go home at the end of the day, you know you've done something amazing," said Martin Molloy, director of the Community Action Center at United Way of Southeastern Pennsylvania, who was among the volunteers. "Just the ability to expose people to these organizations and the work they do, that's what makes MLK Day so powerful."