Many left floral tributes at the castle's gates, and one mourner told Reuters the world had lost a "great man" in the Duke of Edinburgh.
"I couldn't speak highly enough and say enough about him and what he's given to the world," said 52-year-old hospital activity coordinator Leah Dobson.
Philip, Queen Elizabeth's husband who helped modernize the monarchy and steer the British royal family through repeated crises during seven decades of service, died on Friday at Windsor Castle aged 99.
As news broke of Philip's death, radio and television broadcasts were interrupted with the national anthem "God Save The Queen."
Union flags were half-masted at all royal residences and British government buildings.
Philip's sharp wit and dedication to his duty earned him widespread popularity in Britain, but he was also criticized for off-the-cuff racist and sexist remarks.
- We've lost a great man. And he's given so much, so he deserves the tributes-- much, much more tribute than we're actually giving him. So I couldn't speak highly enough and say enough about him and what he's given to the world.
- No. Yeah, he is. Yeah, I know. But it's not letting me.
- He's left so much behind. His legacy-- helping people throughout his life. And he had a great sense of humor. Uplifted everybody.