Will.i.am has led the fierce backlash against Kanye West after he claimed the enslavement of African Americans over centuries may have been a "choice".
The singer said it was "one of the most ignorant statements that anybody who came from the hood could ever say about their ancestors".
He also said Kanye's comments "broke my heart" and were "harmful".
Kanye earlier told TMZ: "When you hear about slavery for 400 years... for 400 years? That sounds like a choice."
He added: "You was there for 400 years and it's all of y'all? It's like we're mentally imprisoned."
He later tweeted to clarify that "of course I know that slaves did not get shackled and put on a boat by free will".
He added: "My point is for us to have stayed in that position even though the numbers were on our side means that we were mentally enslaved."
And then he claimed he was "being attacked for presenting new ideas".
That led to a wave of criticism from fans, fellow artists and others on social media - including the viral hashtag #IfSlaveryWasAChoice.
Asked about the comments on ITV's Good Morning Britain, Will.i.am said: "That broke my heart, because I thought about my grandma, who was born in 1920, and her connection with her mom who raised her, who was born in the late 1800s.
"And my grandmother's grandma, who was a slave. And when you're a slave, you're owned. You don't choose if you're owned. When you're a slave you're deprived of education. That's not choice, that's by force.
"So I understand the need to have free thought, but if your thoughts aren't researched, that is just going to hurt those that are still in conditions where it's not choice."
The musician said it "makes me want to cry that we're even talking about this" when there are problems in the world today that need addressing.
Kanye West stirs up the TMZ newsroom over TRUMP, SLAVERY and FREE THOUGHT. There's A LOT more that went down ... and the fireworks are exploding on @TMZLive today. Check your local listings for show times. pic.twitter.com/jwVsJCMPiq— TMZ (@TMZ) May 1, 2018
Will.i.am also said the comments seemed out of character for the Kanye he knows. "That's not Kanye," he said.
"To me, that's a different person that's saying that, and I hope it's not to raise awareness so you could sell a record and some shoes, because that would be the worst thing to do, to stir up this very touchy race situation and you be the benefactor from it.
"So I encourage you, if you really believe this, give your shoes away for free, give your album away for free. And I don't like talking about going against my community, but that is harmful."
He concluded: "I will not throw my ancestors under the bus to profit."
Others criticising Kanye included film director Spike Lee, who accused him on Instagram of making "uneducated comments" and urged him to "WAKE UP".
Lee wrote: "'SLAVERY... A CHOICE'??? My Brother, OUR ancestors did not choose to be stolen from mother Africa. OUR ancestors did not choose to be ripped of our religion, language, culture.
"OUR ancestors did not choose to be murdered, lynched, castrated, raped, burnt at the stake, families sold apart. OUR ancestors built this country (on land stolen from the Native Americans) from the ground up under the institution of SLAVERY."
On Twitter, musician Talib Kweli, wrote: "I will always have love for @kanyewest but bro out here putting targets on our backs. Slavery was not a choice."
I will always have love for @kanyewest but bro out here putting targets on our backs. Slavery was not a choice.— Talib Kweli Greene (@TalibKweli) May 1, 2018
An African-American history professor at North Carolina State University said in a Twitter thread that Kanye's remarks were "uninformed" and an "embarrrasment".
Haven’t watched the whole Kanye event today, working my way up to it. I will say, that a milder version of the “slavery is a choice” argument is made by uninformed people all the time. I’ve had young men in my courses say “they never would have enslaved me.”— Blair LM Kelley (@profblmkelley) May 2, 2018
Referring to the star's 2004 album The College Dropout, comedian Romesh Ranganathan wrote: "Kanye West is an incredible advert for finishing college."
Musician John Legend, who got into an exchange with Kanye over his support for President Trump last week, retweeted a string of people criticising him.
They included civil rights activist DeRay Mckesson, who wrote: "Kanye's rhetoric continues to fuel the racist right-wing folks who believe that black people are responsible for their oppression."
Kanye’s rhetoric continues to fuel the racist right-wing folks who believe that black people are responsible for their oppression.— deray (@deray) May 1, 2018
However, rapper The Game came to his defence, calling Kanye "a genius". He wrote: "People who've never achieved greatness are not allowed to question it."
Kanye’ is a genius. People who’ve never achieved greatness are not allowed to question it.— Black Jesus (@thegame) May 2, 2018
Is this a marketing stunt?
By Mark Savage, BBC music reporter
Is Kanye West stoking controversy simply to sell records? Yes and no.
The star returned to Twitter two weeks ago after almost a year away. That's pretty standard behaviour - lots of artists "go dark" on social media in the run-up to a new record, only to reappear in (what they hope is) a blaze of publicity when the release date draws near.
But West is instinctively a provocateur. He started to go off-message, tweeting about his admiration of Donald Trump and right-wing commentators who "challenge" conventional thought.
His subsequent statements online and on camera, including the extraordinary assertion that slavery might have been a "choice", have only stoked the controversy further.
But if this is all a marketing ploy, as Forbes suggests, it's backfiring spectacularly. Because unless I'm mistaken, alienating your fanbase isn't a commonly accepted principle of advertising.
All the same, West will use the controversy to fuel his music. After all, the song he released on Saturday, Ye Vs The People, which sees fellow rap star TI challenging his views, was apparently recorded just 48 hours earlier.
So he's reacting and creating in addition to provoking and promoting, which makes this a curiously compelling moment in music.
"I think he's trying to take people on a journey," said US radio host Ebro Darden, after speaking to West last week. "What I've expressed to him is that he better hurry up and get to his destination."
Since Kanye made the comments suggesting slavery was a "choice", a meme has developed on social media mocking the rapper's remarks.
The hashtag #IfSlaveryWasAChoice became the top trending topic in the US just hours after the TMZ broadcast.
Many have used the hashtag to create fictitious scenarios, where enslaved African Americans were working on plantations out of their own free will.
US entrepreneur Luke Lawal imagined a scenario where the cotton pickers have their Mac computer out in the field and are selecting their playlist for the day.
Others imagined a world where they worked hard to get to their dream slave position.