Rich tributes continue to pour in for pop legend Whitney Houston, who died on Saturday in Beverly Hills, California.

Celebrities on Twitter were quick to respond to singer's death.

Russell Simmons said he was "deeply saddened by the death of Whitney Houston...may she rest in peace."

"Just heard the news. so crazy," tweeted Justin Bieber, "One of the GREATEST VOICES EVER just passed. RIP Whitney Houston. My prayers go out to her friends and family I am sadden to hear of the passing of Whitney Houston today..."

Mariah Carey expressed her condolences over a series of tweets, saying: "Heartbroken and in tears over the shocking death of my friend, the incomparable Ms. Whitney Houston. My heartfelt condolences to Whitney's family and to all her millions of fans throughout the world. She will never be forgotten as one of the greatest voices to ever grace the earth."

The two main men in Whitney's life - ex-husband Bobbie Brown and her mentor Clive Davis - both expressed their feelings on stage over the weekend's Grammy ceremony.

Bobbie discovered the news while performing with New Edition at a show in Mississippi. At one point in the concert, a teary Brown shouted "I love you, Whitney!" and blew a kiss toward the sky. While performing "Tenderoni," Brown broke down, sobbing and asked the crowd to sing along, US Weekly reported.

Singer Whitney Houston, who died on February 11, 2012, is shown on a video screen in a 1994 Grammy performance during the 54th annual Grammy Awards in Los Angeles, California, February 12, 2012.Reuters
Members of the audience applaud as singer Whitney Houston, who died recently, is shown on a video screen in a 1994 Grammy performance during the 54th annual Grammy Awards in Los Angeles, California, February 12, 2012.Reuters
Flowers and a card are seen at a makeshift memorial in front of The New Hope Baptist Church, where Whitney Houston sang in the choir, to mourn the death of the singer in Newark, New Jersey February 12, 2012. Houston, whose soaring voice lifted her to the top of the pop music world but whose personal decline was fueled by years of drug use, died on Saturday afternoon in a Beverly Hills hotel room. She was 48.Reuters
Messages are left for Whitney Houston at a makeshift memorial in front of The New Hope Baptist Church, where Houston sang in the choir, to mourn the death of the singer in Newark, New Jersey February 12, 2012. Houston, whose soaring voice lifted her to the top of the pop music world but whose personal decline was fueled by years of drug use, died on Saturday afternoon in a Beverly Hills hotel room. She was 48.Reuters
Civil rights leader Reverend Jesse Jackson speaks with the media outside The New Hope Baptist Church where Whitney Houston sang in the choir to mourn the death of the singer in Newark, New Jersey February 12, 2012. Houston, whose soaring voice lifted her to the top of the pop music world but whose personal decline was fueled by years of drug use, died on Saturday afternoon in a Beverly Hills hotel room. She was 48.Reuters
Flowers and a card are seen left as a memorial to deceased singer Whitney Houston, in front of the New Hope Baptist Church, where she sang in the choir, in Newark, New Jersey February 12, 2012. Houston, whose soaring voice lifted her to the top of the pop music world but whose personal decline was fueled by years of drug use, died on Saturday afternoon in a Beverly Hills hotel room. She was 48.Reuters
Parishioners of The New Hope Baptist Church where Whitney Houston sang in the choir leave after a morning service in Newark, New Jersey February 12, 2012. Houston, whose soaring voice lifted her to the top of the pop music world but whose personal decline was fueled by years of drug use, died on Saturday afternoon in a Beverly Hills hotel room. She was 48.Reuter
People gather at a makeshift memorial on the corner of Santa Monica and Wilshire Boulevard to mourn the death of singer Whitney Houston in Beverly Hills, California February 12, 2012. Houston, whose soaring voice lifted her to the top of the pop music world but whose personal decline was fueled by years of drug use, died on Saturday afternoon in a Beverly Hills hotel room. She was 48.Reuters
Mourners gather at a makeshift memorial on the corner of Santa Monica and Wilshire Boulevards as they mourn the death of singer Whitney Houston in Beverly Hills, California February 12, 2012. Houston, whose soaring voice lifted her to the top of the pop music world but whose personal decline was fueled by years of drug use, died on Saturday afternoon in a Beverly Hills hotel room.Reuters
L'tanya Smith and Ronald Malone of Los Angeles embrace as people gather at a makeshift memorial on the corner of Santa Monica and Wilshire Boulevards to mourn the death of singer Whitney Houston in Beverly Hills, California February 12, 2012. Houston, whose soaring voice lifted her to the top of the pop music world but whose personal decline was fueled by years of drug use, died on Saturday afternoon in a Beverly Hills hotel room. She was 48.Reuters
Parishioners of The New Hope Baptist Church where Whitney Houston sang in the choir leave after a morning service in Newark, New Jersey February 12, 2012. Houston, whose soaring voice lifted her to the top of the pop music world but whose personal decline was fueled by years of drug use, died on Saturday afternoon in a Beverly Hills hotel room. She was 48.Ruters