Joshua, who holds the WBA, IBF and WBO belts, joined a march in his home town of Watford, while three-time Wimbledon champion Becker posted on social media to show his involvement in the big rally in London.
Becker, 52, donned a green face mask, along with a black hoodie and cap, as he mingled among the large chanting crowd in Parliament Square.
The tennis champion, whose first wife of seven years, Barbara Feltus, had an African-American father, has long spoken out against racism.
“Of course there is racism in sport, because racism is part of our society,” he told CNN back in 2012.
Joshua speaks out
Joshua took a more active part in his local demonstration, addressing a tight-knit crowd who had gathered around him. “We stand united against the virus, and the virus I’m referring to is racism,” he said.
“We can no longer, from today onwards, be proud, we can no longer sit back and remain silent on the senseless, unlawful killings, sly racism of another human based only on what? Their skin color.
“We need to speak out in peaceful demonstrations, just like today, so well done, Watford.”
Joshua, who is being lined up for a big money world title unification bout against fellow Briton Tyson Fury, wore a knee brace and was on crutches at the demonstration. That was a “precautionary measure” after he felt a twinge in training, according to his spokesman.