The Duke of Sussex attended his big shindig at St. Paul's Cathedral Thursday to help ring in the 10-year anniversary of his famous charity -- and while he didn't have his dad, brother, or any other working Royal in attendance to help him celebrate ... he still powered through.

Harry showed up with a smile on before and after the commemoration ... and he even did a public walkabout to shake hands and exchange pleasantries with Britons on the ground.

Indeed ... there were quite a few people who arrived to give Harry his flowers -- and he even received lots of happy cheers from the crowd, as well as messages of congratulations. When he went around to wave hello and talk to folks, he looked pretty damn happy himself.

While it might not seem like much on its face ... it's actually telling, especially since King Charles -- who said he wouldn't be able to meet with Harry during his time in London -- was hosting his own bash around the same time Thursday ... where he was shaking hands too.

Worth noting ... there weren't any senior Royals in attendance for Harry's thing, but they definitely showed up for Charles. People of note who did go to Harry's event, however ... his extended family on his mother's side -- who put on a united front as they hit the church.

Again ... Harry put out a statement saying he understood his father was swamped and unable to get together -- and as polite as he's being publicly ... ya gotta imagine this hurts a bit.

It goes without saying ... their relationship has been strained in recent years -- especially after Harry abandoned his Royal post and flew on over to California with Meghan Markle to start a new life.

The saga is well-documented ... and while Harry and Charles have met and seen each other sporadically since the dust settled on all that -- it's clear they're still not the closest right now.