The Wire co-star Clarke Peters told BBC Radio 4's Today programme: "He was a generous actor. It wasn't me, me, me. It was us, us, us."
Film-maker Spike Lee described Williams on Instagram as "his brother" and said he "was shook" at the news.
The Wire creator David Simon said he was "a fine man and a rare talent".
Alongside a picture of Williams, Simon wrote that he was "too gutted right now to say all that ought to be said", adding: "On our journey together he always deserved the best words. And today those words won't come."
The star was found dead in his New York apartment on Monday. US media quoted law enforcement sources as saying he died from a suspected drug overdose, but that has not been confirmed.
Williams, who was nominated for three Emmys, had openly discussed his struggles with drugs over the years.
In The Wire, he played Omar Little, a gay, streetwise robber of drug gangs.
'Sad and angry'
He was also known for playing gangster Albert "Chalky" White in the Prohibition period TV series Boardwalk Empire. Other TV credits included Lovecraft Country, The Night Of, Bessie and When They See Us.
Peters, who played Lester Freamon in The Wire, said he "didn't know whether to be sad or angry" about the death of his co-star.
"I'm sad that Michael is no longer with us because there was a very promising personality that could have done so much more. I'm angry because I feel he's let us down by going back when he could have reached out," he told the Today programme.
"Once a person is on that path, maybe it is very difficult for them to get off. So we, as a society, have to pull our socks up so we can help people like this."
'He sings their truth'
Spike Lee said: "Whew lawd, dis is a tough day -1st we lose the great French actor Jean Paul Belmondo and now my brother Michael K Williams.
"Our greats are leaving us left and right. God bless," he added alongside a photograph of himself with Williams at a basketball game.
Other tributes from the world of entertainment came from the likes of his The Wire co-star Wendell Pierce.
"The depth of my love for this brother, can only be matched by the depth of my pain learning of his loss," Pierce tweeted.
"A immensely talented man with the ability to give voice to the human condition portraying the lives of those whose humanity is seldom elevated until he sings their truth," Pierce added.
Oscar winner Viola Davis simply wrote: "No! So sad. Man do I wish this were a different ending. Rest well. Praying for your loved ones."
Fellow actress Kerry Washington posted on Twitter: "So heartbroken. Thank you for the beauty and joy you brought to the world. Sending love and prayers to your friends, family and everyone who adored you me included. We adore you Sir."
She also retweeted a video of Pierce with Williams as the former praised the latter's acting ability on the red carpet.
"This is heartbreaking to learn. We have lost an enormous talent in Michael K Williams," tweeted Star Trek actor George Takei.
Director James Gunn described Williams as one of the "most gentle souls I've ever met".
In a statement, the HBO TV network, which aired 60 episodes of The Wire between 2002 and 2008, said: "We are devastated to learn of the passing of Michael Kenneth Williams, a member of the HBO family for more than 20 years.
"While the world is aware of his immense talents as an artist, we knew Michael as a dear friend who was beloved by all who had the privilege to work with him. We send our deepest condolences to his family for this immeasurable loss."
Omar Little, a TV game-changer
Michael K Williams' complex portrayal of Omar Little, a gay, shotgun-toting robber of drug dealers, helped cement The Wire's game-changing depiction of life in the projects of Baltimore.
Omar represented the duality of a black experience never before given such honest screen time. Nowhere was that better represented than his exchanges with Wendell Pierce's detective Bunk, a black schoolfriend who reminded him of the different paths life afforded them.
Interviewed together years before Williams' death, Pierce said his co-star's performance gave voice and flesh to characters "that most people would have never given the same humanity to… opening a window to a world of men that we pass by or don't know about".
Pierce described the scenes they shared as the "greatest of my career", performed by a man who contributed two of the "most iconic characters in the history of American television" in Omar Little and Chalky White, and no doubt could have given many more.
Born in 1966, Williams had a turbulent upbringing in Brooklyn, New York. Despite trying to avoid confrontation, he later said he "had a way of always finding myself in trouble - like, big trouble, you know what I'm saying?"
He began his career as a dancer and performed with Madonna and George Michael before moving into acting with companies including the prestigious National Black Theatre Company.
He got the distinctive scar on his forehead and right cheek after being slashed by a razor on his 25th birthday - but that inadvertently led to his first acting job after Tupac Shakur saw a photo of him and decided he looked "thugged out enough" to play his brother in the 1996 film Bullet.
In a BBC interview in 2018, Williams explained why The Wire was such a success. "I think it has just stayed with people because it's still relevant now, because of the climate of what's going on in the United States. I think it's actually a little sad," he said.
"It was the first show I saw that was created as what I call 'edutainment'. It dived so honestly into what was wrong in our society, from the police department to our lawmakers to our school system, and the media. It represented what was happening in our community."
But in 2011, he told The Hollywood Reporter he was "overwhelmed" by the success of season one and went "into an emotional tailspin" during season two.
"I wasted a lot of money, I started drinking and drugging again, and, at the end of season two, I was forced to move back to my projects and slept on a mattress on the floor."