Dropping onto both knees, Wonder lead a prayer for "out planet, our future, our leaders of the world, and our globe".
"Tonight, I'm taking a knee for America. I wanted to say that prayer before I serve you my musical meal". "He's fired. He's fired!'" Trump said at an Alabama rally on Friday in support Luther Strange, who is running in a special GOP primary election next week.
The President continued: "You know, some owner is going to do that".
Colin Kaepernick #7 and Eric Reid #35 of the San Francisco 49ers kneel in protest during the national anthem prior to playing the Los Angeles Rams in their NFL game at Levi's Stadium on September 12, 2016 in Santa Clara, California.
Maxwell, who is African-American and from a military family, earlier criticized the president on Twitter.
Wonder's hopeful opener comes hours after Trump called for a football boycott and urged National Football League owners to fire players with the gall to protest social injustice issues, such as Kaepernick.
Then he put his other knee down. "All of us are valuable and important and must take care of each other and this planet", Wonder said.
At the Global Citizens Festival, Wonder also made a thinly veiled comment about Trump's handling of tensions with North Korea, saying, "weapons are real and rhetoric is unsafe".
Wonder then went on to entertain festival-goers with performances of his hits including Signed, Sealed, Delivered I'm Yours and Sir Duke - and a cover of John Lennon's Imagine.
Wonder also shared a brief message condemning bigotry and sexism ahead of Saturday's concert. A number of players took to their knees and linked arms in solidarity during the Ravens and Jaguars game in London on Sunday.