7:40 AM PT -- Airport officials just said, "St. Louis Lambert International Airport Police arrested a 51 year old airline pilot Wednesday morning for unlawful use of a weapon after TSA officers discovered a loaded 9 mm pistol in his carry-on luggage." The official says the pilot did not have any authorization to carry a firearm.

6:49 AM PT-- A spokesperson for the St. Louis Airport tells TMZ only one pilot was arrested. As we reported, 2 eyewitnesses tell us they saw 2 pilots in handcuffs.

Two Southwest Airlines pilots were arrested Wednesday morning at St. Louis Lambert International Airport ... eyewitnesses tell us they brought guns to the airport.

An eyewitness who was at the airport tells TMZ the pilots were at the TSA security checkpoint when 2 law enforcement officers handcuffed both pilots. A TSA employee told the eyewitness the pilots were arrested because they had firearms in their carry-on.

One of the arresting officers told one of the pilots, "We don't mess around here."

The passengers on flight #1717 bound for LAX were told to deplane and they each received a $100 voucher.

We reached out to Southwest ... so far, no word back.