The crowd had come up the Peninsula on the Freedom Train, a specially chartered Caltrain devoted to celebrating King's legacy.
One of the riders, Jim Zito, 54, of San Jose, said he brought his 10-year-old son to the march with him to show the importance of what King stood for, and how to exercise civic rights.
"What we do today can affect the future," he said.
Zito was 10 years old himself when King was assassinated.
The crowd, which included San Francisco Police Chief Greg Suhr, was made up of people of all ages, and the marchers made their way to Yerba Buena walking up Third Street in the sun. Those in the front sang songs, including "Amazing Grace."
Many carried signs, one reading "Say no 2 racism and sexism." Another read, "Jobs, housing and education, not war and occupation."
The march is culminating in a celebration at Yerba Buena Gardens.