EXCLUSIVE

Justin Bieber's got less respect for the little people than Godzilla ... so says a rep for the biggest janitor union in the country, after TMZ posted video of the singer taking a leak in a restaurant mop bucket.

A rep for 32BJ SEIU -- aka the largest property services workers union in the US -- tells TMZ, “Unfortunately, people are often oblivious to the fact that workers like our members are faced daily with the consequences of such thoughtless acts."

The rep adds, "[Bieber's behavior] serves as a stark reminder of the essential jobs that building workers perform and the very difficult conditions under which they perform them. We welcome a heightened awareness of workers’ dignity among young people, and hope many will be moved to work for economic justice.”

Maybe Bieber should suit up for a day or two and see what it's really like to clean up after spoiled little twerps.

Here's the pee bucket video again for good measure.