Martin O'Malley

"I'm doing it because O'Malley didn't do nothing for Baltimore," said Duane Davis of Baltimore Bloc, a group that was involved in protests following Gray's death. "He created a prison system in Maryland. That's his legacy."

Davis has a colorful history in Maryland and elsewhere, and has been a longtime critic of O'Malley. He was charged in 2011 for leaving a toilet rigged with electronic devices outside a former courthouse in Towson (police thought it was a bomb). That same year he sued O'Malley, but his case was quickly dismissed.

Election records show Davis filed as a Republican candidate for lieutenant governor in 2014 but was later disqualified.

Davis declined to say how many protesters he expects to attend on Saturday. The group plans to march from Federal Hill, where O'Malley will make his announcement, to City Hall. Protesters will then feed the homeless at a shelter, he said.