"And we'll go up, up, up, but I'll fly a little higher," 10-year-old Zachary Le, of Brooklyn Park, sang along with others from his school, from other schools, churches and groups, along with individuals and families.

Le's voice was among thousands who sang "Clouds" in the Mall of America's rotunda Thursday evening. The song was written by Zach Sobiech, an 18-year-old Lakeland teen who died of cancer in May.

Mall officials estimated that 5,000 people circled the rotunda, on four floors, singing along with a video viewed by millions worldwide of Sobiech strumming his guitar and singing "Clouds." In it, he sang his goodbyes while dying of osteosarcoma, a bone cancer that strikes kids.

Thursday, KS95 (94.5 FM) sponsored the "World's Largest 'Clouds' Choir" as part of its radiothon benefiting Children's Cancer Research Fund and Gillette Specialty Children's Care.

From Delano High School to the Twin Cities Gospel Choir, people sang together near two towering tinsel Christmas trees as streamers cascaded down upon them.

"Zach Sobiech, he was a really kind man, and he taught me to live life to the fullest," Le said as he stood in a blue T-shirt nearly reaching his knees and emblazoned with the song's refrain "Up, up, up."

From tiny tots to Sobiech's great-grandmother, people of all ages wore those T-shirts while joining in a rendition of the song that his parents, Laura and Rob Sobiech, said they found full of hope and joy.

"The goal is kind of a pay-it-forward," Rob Sobiech said after the concert. "Zach isn't here anymore, but the community can help with solving children's cancer."

Sobiech died May 20 after a four-year fight against the disease. In his last months, he turned to his guitar and wrote songs to say goodbye to those he loved with the help of longtime pal Sammy Brown.

Laura Sobiech said Zach intended "Clouds" for his family and friends, but then it took off on the radio, "and it just kept growing." The award-winning music video has had more than 8.3 million views on YouTube worldwide and was a top iTunes seller. Media firm SoulPancake's documentary "My Last Days" has been seen more than 11.4 million times, http://tinyurl.com/c35ry78.

"It just really speaks to children, and that's a blessing because Zach died of children's cancer," his mother said. "What it boils down to was that Zach was a kid in a hopeless situation who brought hope to people."

So far, the osteosarcoma research fund he started has raised about $625,000, she said.

Thursday's event alone, with 3,500 registered and hundreds more just showing up, raised about $5,000 as part of the radiothon, which is expected to raise more than a half-million dollars this weekend, said Dan Seeman, the station's general manager.

Among those singing Thursday evening was Amanda Woodward, 17, of Stillwater, who mused on what her friend Zach Sobiech would have thought of the giant "Clouds" choir.

"He would have been up there smiling," she said.