Jay-Z Stops UK Show to Sing 'Happy Birthday' to Beyoncé

Jay-Z fans may have come to see him perform ... but they ended up being performers themselves -- because he had them serenade Beyoncé!

The rapper played at Tottenham Hotspur Stadium in London on Friday as part of his limited tour engagement celebrating the 30th anniversary of his album "Reasonable Doubt" and the 25th anniversary of "The Blueprint."

In the middle of the show, Jay-Z stopped ... and told the crowd he wanted them to sing 'HBD' to his lady -- who turned 45 on Friday.

Check out the vid ... fireworks start exploding, and the band strikes up a song -- before thousands of voices scream out the track with Jay leading them.

Bey looks overjoyed ... clearly shocked about her man's grand romantic gesture. At the end, the lights dimmed, and Jay transitioned into his track "N****s in Paris" -- a pretty crazy transition, we gotta say.