Stevie Wonder has called on fans to "honour the principle of peace and unity" while marking Martin Luther King Jr. Day.

In a four-minute video posted to Twitter on Monday, the singer-songwriter observed the federal holiday in the U.S. by asking his followers to consider whether they ever take on board the late civil rights activist's teachings.

Wonder explained how four decades ago he and others were "marching in the cold and snowy streets of Washington, D.C." before "peacefully entering the Capitol" to explore ways to reach across the aisle.

He emphasised the word "peacefully", seemingly referring to the attack of the U.S. Capitol building by supporters of former U.S. president Donald Trump on 6 January 2021.

"Forty years, what have we done for the planet? How have we really helped each other? Where has poverty been eliminated, why are guns still protected, and why has hate been elevated?" Wonder commented.

"Truthfully, we've done very little in 40 years," the 72-year-old argued.

Stevie added, "Now it is time for us to grow the (f**k) up, and get out of our virtual delusions that sell murder, mayhem, terrorism, and hate... Dr King, I wish I could say you were here. But it feels like we did not deserve you then, and we're not much better now."

Continuing, he added, "And people... I believe deep in my soul the universe is watching us - and she is p**sed. But will we fix it? I hope so."

Martin Luther King Jr. Day was signed into law in 1983.

The activist was assassinated in 1968 at the age of 39.