Los Angeles (CNN) -- Word of Casey Kasem's death hit Sunday morning -- the same time of day that millions of people listened to his "American Top 40" radio show each week for four decades.
Ryan Seacrest, who took over hosting duties from Kasem 10 years ago, called it "a sad day for the broadcasting community and for radio listeners around the world."
"When I was a kid, I would listen to Casey Kasem's "AT40" show every weekend, and dream about someday becoming a radio DJ," Seacrest said in a statement. "So when decades later I took over his AT40 countdown show, it was a surreal moment. Casey had a distinctive friendly on-air voice, and he was just as affable and nice if you had the privilege to be in his company. He'll be greatly missed by all of us."
Seacrest will expand on that in next weekend's AT40 show, which will be dedicated to the "King of Countdowns." The show's website posted an audio tribute to Kasem Sunday so fans could hear his voice.
DJ Rick Dees, whose own syndicated music countdown competed with Kasem's, told CNN that their "friendly competition led to years of friendship and respect. We will all miss his style, his voice, and his ability to communicate. Casey was an original who can't be replaced."
Legendary radio DJ Bruce Morrow, known professionally as "Cousin Brucie," told CNN that Kasem was "one of my mentors and a very sweet, unassuming guy."
"I feel so bad," Morrow said Sunday. "I've been watching this reality show around Casey unfold over the past few weeks. I'm upset the way his life went down at the end. But as a radio icon, Casey was important in setting the stage for many of us. With 'American Top 40,' he was an innovator and the first who took modern, contemporary music and translated it into America's Top 40 hits. He showed us the way. Casey was 'The Professor' of Top 40 radio. He and his work opened doors for radio personalities and music programming."
Many of the tributes posted on social networks Sunday included variations on Kasem's traditional closing line: "Keep your feet on the ground and keep reaching for the stars."
Marie Osmond, whose music Kasem introduced many times in the 1970s, tweeted that he "Chngd the industry w/AT40 Cntdwn."
We've lost a music industry icon. Chngd the industry w/AT40 Cntdwn. RIP Casey Kasem. "Keep ur feet on the grnd & keep reaching 4🌟s"— Marie Osmond (@marieosmond) June 15, 2014
KISS lead singer Paul Stanley tweeted that Kasem "brought music and his joy to us all. A good man."
Motley Crue bassist Nikki Sixx, who co-hosts a radio show with Kasem's daughter Kerri, tweeted that Kasem "inspired all of us in radio & turned millions of people onto music."
Nancy Sinatra, daughter of music legend Frank Sinatra, said in a Twitter posting that Kasem was "one of the most decent men I ever knew."