James has been an outspoken critic of President Donald Trump and spoke at length about his social activism at the Cavs' media day.
"I will in my voice, I will in my passion, I will in my money, I will in my resources to my youth and my inner city and outside my inner city to let these kids know that there is hope, there is greater walks of life, and not one individual, no matter if it's the president of the United States...can stop your dreams from becoming a reality," James said.
During the national anthem before the game, all Cavaliers players stood while linking arms, while the Celtics players stood as normal.
In September, after NFL players sparked debate within that league over protests during the national anthem, NBA commissioner Adam Silver said he expected NBA players to stand during the national anthem.
"We have a rule that requires our players to stand for the anthem," Silver said. "It's been our rule as long as I've been involved with the league, and my expectation is that our players will continue to stand for the anthem."