Steve-O Jumps off Tower of London Bridge, Detained by Police |

LONDON LEAP

Steve-O taking a plunge from one of London's famous landmarks has him in hot water with cops ... who were waiting for him moments after he splashed into the River Thames.

The "Jackass" star tells TMZ ... he's in England to tape his third comedy special, so on Thursday he jumped off the Tower of London Bridge and was promptly detained by cops.

TMZ obtained video of the crazy stunt, and you see Steve-O counting down in front of a crowd before jumping off the bridge ... all while decked out in all sorts of Union Jack gear, including an umbrella.

Steve-O says he spent a good portion of the afternoon in a police car and says cops told him their issue with his stunt is it might encourage others to make suicidal leaps from the famed bridge.

In the end, Steve-O says police let him go, and, as far as he's concerned, the craziest part wasn't the jump ... it's the state of the Thames.

Steve-O says the river was absolutely filthy ... which probably isn't a great combo with the new big tattoo he's sporting on his leg. Dirty water isn't exactly great for open wounds.

The bridge jump wasn't Steve-O's only stunt of the trip ... he says he also jumped off one of those famous double-decker buses, which he considered much worse than the bridge jump. Yeah, pavement makes for a tougher landing than water.