Prince George's County Fire officials were called to The Joker's Jinx roller coaster at the park just before 2:30 p.m. Sunday. No injuries have been reported.
Assistant Fire Chief Paul Gomez says the riders are sitting upright near the top of the ride. He says it will likely take a few hours to get them down.
It's not clear what caused the ride to get stuck. Telephone and email messages left for a Six Flags spokeswoman were not immediately returned.
Six Flags' website says the ride goes 60 miles per hour and upside down four times.