Surprises Crowd At MLK Jr. Day March

Brittney Griner, less than 6 weeks after she was sprung from a Russian penal colony, surprised a group of people gathered for an MLK day march in Phoenix ... on what would have been Martin Luther King Jr.'s 94th birthday.

The Phoenix Mercury superstar hit up the event with her wife, Cherelle ... and took pics with fans, who were visibly elated to see the hooper.

Of course, states celebrate MLK Jr. Day on the third Monday in January ... a day to remember one of the most influential civil rights leaders in the history of the world.

FYI, Arizona was the last state to recognize MLK Jr. Day (1992) after President Ronald Reagan signed a law making it a federal holiday in 1983 (though it was first…