Check out this clip -- Mark’s in full-on brawl mode on top of a double-decker bus with Kit Harington's character, who suddenly grabs Mark's on-screen wife, Michelle Monaghan, and it’s game on!

Mark’s hands are up, telling Kit to chill out as Michelle’s held at knifepoint. Tension is thick until Mark pulls a move that frees Michelle -- right into his arms.

The cast was in full action mode, while the iconic red bus cruised through Central London as passersby got an unexpected treat, spotting big-name stars in the middle of a Hollywood production.

Kit’s character is still under wraps, but it looks like he’s playing the villain in the sequel, which is about an ex-government operative forced to flee with his family after being attacked.