"If he could see for himself that all of you have showed up to show my brother love at this hard time, and the family gets to see all of you show the love that you've shown Paul, it's going to stay with me forever," the actor, 46, told the crowd over a police car's loudspeaker in a video posted by KTLA-TV.
"I just wanted to say thank you," he added. "Thank you for coming down here and showing that angel up in heaven how much you appreciated him."
Diesel also honored Walker, who died at 40 in a fiery car crash Saturday in Santa Clarita, Calif., with a remembrance on Facebook.
Under a photo of himself and Walker, Diesel wrote: "Pablo, I wish you could see the world right now… and the profound impact, your full life has had on it, on Us… on me…"
Using Walker's character's name from The Fast and the Furious movies, he added: "I will always love you Brian, as the brother you were… on and off screen."
Diesel added a quote from Scottish poet Thomas Campbell: "To live in the hearts we leave behind, is not to die."
The investigation into Walker's death continues, as police look at speed as a possible factor in the crash. Autopsies are expected to take place Tuesday on the remains of Walker and the driver of the car, Roger Rodas.
Walker's family on Monday posted a statement to the late actor's Facebook page thanking fans for their support.
"Paul Walker's family appreciates the outpouring of love and goodwill from his many fans and friends," it read. "They have asked, in lieu of flowers or other gifts, that donations please be made to Paul's charity Reach Out Worldwide."
The statement continued: "It's comforting for them to know that the son, brother, and father they love so much is equally adored, respected, and appreciated by so many. Paul founded the organization with the genuine desire to help others, and it's important to his family to keep his memory alive through ROWW."