"He invited me to be up here, and I respectfully declined because the organizers had worked for years to get this together, and I hadn't done anything."
Like many others before him, Russell said the fight for civil rights that King led is not over.
"From my point of view, you only register progress by how far you have to go," he said. "I'm here to join you and to implore you: The fight has just begun.
"We can never accept the status quo until the word 'progress' is taken out of my vocabulary."
Russell, who won 11 NBA championships with Boston and turns 80 in February, seemed at ease speaking before thousands of people. He joked as he started his remarks: "I was sitting in the first row 50 years ago, and it's nice to be anywhere 50 years later."
President Obama is scheduled to speak at around 3 p.m. Wednesday.