Tom Cruise Goes Clubbing in London ... to Surprise His DJ Son

Tom Cruise crashed the London club scene last night ... but he wasn't there to party ... he was there to support his son who was DJing at one of the biggest nightclubs in town.

Cruise rolled into Chinawhite ... where a TON of Olympians partied during and after the games. It's also where Tom's 17-year-old son Connor -- aka DJ C-Squared -- was set to perform Thursday.

Sources tell us ... Tom wanted to surprise Connor ... and arranged to quietly enter the club through a neighboring souvenir shop so Connor couldn't see him coming.

We're told Connor LOVED the surprise ... and the two hugged it out and talked before the set.

Sources close to Tom tell us ... Cruise spent 2 hours in the VIP area ... and raved about how "very proud" he is of his son.