TMZ Celebrity Tour bus riders were in a huge treat to end the week — ’cause Andrea Bocelli was in town … and he came aboard to put his pipes on display for one lucky rider!

We ran into the legendary Italian tenor out in the streets of Hollywood Friday — where we happened to be doing our TMZ Celebrity Tour … and one of our guides spotted Andrea and his son, Matteo, in the heart of the city, opting to hop off and interview him.

It made for a pretty epic conversation if we do say so ourselves. Check it out … not only did AB give us the time of day — telling us all about his upcoming projects — but he also obliged us by coming on the bus and chopping it up with the riders.

The dude was super sweet … and once he was on the bus — he did someone a huge favor.

He didn’t do “Ave Maria” here — but Andrea did sing the next best thing … and that would be “Happy Birthday” to one of our tour guide guests. Of course, phones were out for this one.

Just goes to show … you really do see famous people on the TMZ Celebrity Tour — and it’s 100% worth its weight in gold, especially when awesome stuff like this happens. We’ll admit … even we were a little star-struck with Andrea’s pop-in here, ’cause the dude’s an absolute icon.

Thanks for the visit, Mr. Bocelli!