John Krasinski, Rainn Wilson and Jenna Fischer were among the castmembers who headed to the city of Scranton - where the sitcom is set - for the big event, which featured question-and-answer sessions and a parade through town.
After the parade, the stars took the stage for a karaoke concert, performing tracks like Girls Just Wanna Have Fun and Don't Stop Believin', and signed autograph for locals.
Actress Ellie Kemper told Reuters, "I've never seen anything like this. I'm still soaking it in. It's like The Rolling Stones came here or something. I told someone it looked like the parade scene in Ferris Bueller's Day Off."
It was the first time the cast had met up since wrapping production in March (13) - and Carell, who left the show in 2011 to focus on his film career, capped off the celebrations when he showed up unexpectedly to a party held at the nearby PNC Field.
He told the crowd, "Thank you, Scranton. This all is because of you."
The show goes off the air after the finale on 16 May (13) following nine seasons.