Mark Cavendish -- one of the greatest road sprinters of all time -- says he's in pain after a violent spill during an event in Belgium ... leaving him with 2 broken ribs and a collapsed lung.
The crash happened at the Six Days of Ghent competition Sunday night ... when the 36-year-old -- alongside his teammate -- collided with cyclist Lasse Norman Hansen.
You can see in the footage, a biker ahead of Cavendish and Hansen lost control ... causing them to crash into each other at full speed and go spinning on the track floor.