'Foster the People' Singer -- KICKED OUT of Prince Concert

Prince DOES NOT mess around when it comes to cell phones at his private shows ... and had the lead singer of Foster the People KICKED OUT last night when he violated the rule.

TMZ was shooting footage when Mark Foster -- the guy who sings the annoyingly overplayed "Pumped Up Kicks" song -- was escorted out of The Sayers Club when someone saw him tooling around on his iPhone.

Foster begged and pleaded with security to let him back inside -- using every tactic in the book:

-- I have a tab at the bar I didn't close!
-- I promise I wasn't taking a picture, check my phone!
-- I'm a musician, I understand ... I won't do it again.

... and of course, "I'm Mark Foster ... from Foster the People."

After a couple of minutes in limbo, security gave Mark a break and let him back inside ... but only after they MADE HIM check his phone.

On his way out of the show, Mark told us it was a big misunderstanding ... he was simply checking a text message from a friend when he was tossed.