Joshua, who holds WBA, IBF and WBO belts, joined the march in his hometown of Watford, while three-time Wimbledon champion Becker posted on social media to show his involvement in a major rally in London.
Becker, 52, donned a green mask, black sweatshirt and hat as he hung out among the big singers 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 sports, because racism is part of our society,” he told CNN in 2012.
Joshua took a more active part in his local demonstrations, addressing a close audience that gathered around him. “We stand united against the virus, and the virus I mean is racism,” he said.
“We can no longer, from today onwards, be proud, we can no longer sit and be silent about senseless, illegal murders, the cunning racism of another man based only on what? Their skin color.
“We need to talk in peaceful demonstrations, as we do today, so well done, Watford.”
Joshua, who was lined up to unite the world champion with big money against Briton Tyson Fury, wore a knee bracelet and was on crutches at the demonstrations. It was a “precautionary measure” after he felt a twitch in training, his spokesman said.