John Legend has clapped back at a critic who questioned why he publicly denounced R. Kelly in a documentary but never did so for disgraced producer Harvey Weinstein.

The All of Me singer was the most high-profile star to appear in the recent Lifetime docuseries Surviving R. Kelly, which chronicles the sexual abuse allegations levelled against the R&B singer.

On Monday (07Jan19) a user shared an old photo of John and his wife Chrissy Teigen posing with Weinstein and asked why John was happy to denounce R. Kelly but didn't do the same for the movie mogul, who has been accused of sexual misconduct by multiple women.

"I took a photo with and worked with Harvey on several occasions before his abuse was known to me and the rest of the world," John replied on Twitter. "Since his being exposed, his company and career have rightfully been destroyed and he's been indicted. Sounds like something that should happen to R Kelly... If y'all wanna cape (show support) for R and discount all these women's stories, just say it. Don't bring up some old pics of me and somebody else."

Weinstein's career has been left in tatters since the allegations surfaced and he is facing sexual assault charges in New York, while Kelly remains a working musician.

In the documentary, John can be seen saying, "Time's up for R. Kelly... (He) has brought so much pain to so many people."

When he received praise for appearing in the documentary last week, the singer replied, "It didn't feel risky at all. I believe these women and don't give a f**k about protecting a serial child rapist. Easy decision."

Celebrities have reacted to the series on social media, with rapper Master P blaming the parents of women involved in Kelly's alleged sex cult for letting the abuse go that far.

"Ain't nobody gonna play with my kids. Straight up," he said in an Instagram video. "You shouldn't have even been on TV... I don't know if R. Kelly did it or not, that ain't any of my business, I ain't judging, but I'm just saying those parents shouldn't have let that go that far. ... If my kid in there, it’s gonna be all-out war."

Meanwhile, accuser Asante McGee has challenged Kelly via to take a lie detector test on live TV to prove he's telling the truth. A Facebook page titled Surviving Lies was set up over the weekend to discredit accusers like McGee, but it has since been removed.

Both Kelly and Weinstein have denied allegations levelled against them.