Krysten Ritter -- Don't Trust the B--- on United Flight 841

Krysten Ritter was involved in a nasty verbal clash with a male flight attendant Friday that left her fighting back tears ... witnesses aboard the flight tell TMZ.

According to the witnesses, Ritter was on a flight from NYC to LA that was experiencing severe turbulence. We're told at one point Ritter -- whose show "Don't Trust The B---- in Apt 23" premiered this week -- got up when the "fasten seatbelt" sign was on ... and she was immediately confronted by a male flight attendant.

Our sources say the flight attendant was "very aggressive" and yelled at her to sit back down. We're told Ritter was so distraught by the confrontation, she was on the verge of a tearfest.

According to our sources, the supervising flight attendant got wind of what went down and spoke with Krysten. We're told that flight attendant apologized and told Krysten she felt the male flight attendant was out of line and that disciplinary action would most likely be taken.

We spoke with Krysten once she landed -- take a look.