"We are gearing up, and we are growing," Pastor Leslie Callahan of Philadelphia's St. Paul's Baptist Church told thousands at a pre-march rally outside School District headquarters at 440 N. Broad St. "You have not heard the last of us."
On the sound system, Marvin Gaye demanded, "What's going on?" and Public Enemy exhorted, "Fight the power."
One speaker railed against the police, saying they had beaten his son.
"Hands off my children," he yelled. "Hands off your children!"
The march stopped traffic on Broad and then, as demonstrators moved out, closed the area around City Hall and east on Market Street to Fifth Street, site of a concluding rally and speeches.
"This is what democracy looks like," marchers chanted as they moved.
Toward the end of the event, a small group of people used bullhorns to shout expletives at police officers.
"Whoever raised you is s-!" one man yelled at a cop. The officer gave him a thumbs up.
Generally, the tone was upbeat and nonconfrontational. Police, who maintained a heavy presence all day, reported no arrests.
Police officials estimated the crowd at 3,000, while organizers said it was closer to 6,000. Reporters on the scene estimated 5,000.
Organizers with the MLK DARE coalition said interest in the march was driven by coincident events: the killing of African Americans by police, the arrival of a new Pennsylvania governor, and a sense of dashed hope attached to King's birthday.
Groups such as Ferguson Action say King's memory has been clouded by attempts to "soften, sanitize, and commercialize it," reducing blood and sacrifice to "images of men in suits."
Rallies and marches to "reclaim King's legacy" took place in cities around the United States.
"To us, Dr. King was a neatly packaged image of a man who used civil disobedience to barter peace between the races," said Sabrina Sample, one of the Philadelphia demonstrators.
"To many of us, this movement came to a screeching halt. We had won," said Sample, who was born in the 1990s. "But in our ghettos, and even in the suburbs where some of us lived out parts of the American dream, we were reminded of our color and that we lived in what Dr. King called two Americas."
The Philadelphia protest involved more than 70 faith, student, and union groups, springing from last month's "Black Lives Matter" demonstrations. Those protests were propelled by the tension that erupted with the police killings of Michael Brown in Missouri and Eric Garner in New York.
Protest leaders here said the "MLK Day of Action, Resistance and Empowerment" aimed to challenge flash points such as education funding, the minimum wage, and police "stop and frisk" tactics. The march was a model of diversity - young, old, black, white, and Asian. People rode bikes and pushed strollers, walked arm in arm or alone.
"I'll never give up the fight," said Ron Raz, who described himself as a veteran of protests of the 1960s.
Traditionally, King's Birthday in the region has centered on service projects. On Monday, tension between the service-minded celebrants and protest groups was evident as the 20th annual Day of Service got underway at Girard College.
"Some would suggest that what you are doing is somehow 'sanitizing' the work of Martin Luther King," service-day founder Todd Bernstein told volunteers.
In fact, he said, their service paid tribute to King's life and legacy.
The morning at Girard included speeches, music and dance performances, and a health and jobs fair, all of it but a sample of 1,800 projects taking place in schools, neighborhoods, and community centers across the region.
This year's observance celebrated the 50th anniversary of the Voting Rights Act, and many projects centered on voting and registration. It also marked the 50th anniversary of King's visit to Girard, when he met with protesters seeking to desegregate the school.
In the cavernous Girard armory, Brian Leffler and his 11-year-old son, Max, helped make polling-place signs in 10 languages.
"It's fun," said Max, a student at Wilmington Friends School, as he peeled the backing off a huge adhesive strip that read "Vote Here" in Bahasa Indonesia.
Mayor Nutter reminded the Girard audience of the historic march from Selma to Montgomery, Ala., that preceded the passage of the Voting Rights Act in 1965.
"Black lives do matter," he said. "White lives matter. . . . All lives matter."
On Broad Street, a brilliant sun that shone as marchers began to gather soon slipped behind clouds, the sky turning gray and spitting snow. No one seemed to mind.
"Honey, this is important," said Debra Scott, who planned to cover the 1.3-mile route with the help of her cane. "The Lord will carry me."
She wore a long wool coat, gray scarf, and leopard-print hat, believing the discomfort of the cold mattered little compared with the weight of the moment.
"We've got to stand up for our rights," she said. "We can't just talk about it."
Nearby, in a wheelchair, Shawn Tucker was eager to join the march. "It's overdue," she said.
Above the heads of the crowd bobbed dozens of signs and banners, held aloft by Quaker groups and environmental activists, by people demanding higher pay and others insisting on an end to police violence.
"Indict the system," one sign read. "Money for jobs, not war," read another.
Antoine Little, a business manager for Local 427 of the city sanitation workers, held a sign that seemed familiar from another age: "I am a man."
The march, he said, echoed the determination of King and his supporters, a modern undertaking to "teach the young what it means to be African American and what it means to speak up."
"Our communities are hurting," he said. "We need help. But nobody is willing to help us. So we've got to help ourselves."
The march stretched for blocks, at one point reaching from City Hall to Hahnemann University Hospital. It circled City Hall to the west, then headed east on Market. It moved past doormen at hotels, homeless men begging for coins, and perplexed tourists at the Hard Rock Cafe.
People took photos with cellphones. News choppers hung overhead. On the ground, dozens of yellow-jacketed bicycle officers led the route.
"I'm marching today for the lives that we have lost over the years and all of our children that have died," said Sonia Blount of Mount Airy.
"I'm here to march for justice and equality for all humanity," said the Rev. Jeffrey Jordan-Pickett.
At Independence Hall, the protest closed with prayers from Jewish, Muslim, and Christian clerics, and with the ceremonial head-shaving of two women who surrendered their hair in protest of police violence.
"Look at your neighbor. Look at your other neighbor," one of the organizers, the Rev. Mark Tyler, told the crowd. "This is what Philadelphia looks like. . . . And we're here today because Philadelphia has some unfinished business with its residents."
Contributing to this article were Inquirer staff writers Martha Woodall, Jeremy Roebuck, and Mike Newall.