Arnold Schwarzenegger‘s known for his gun-toting characters … but it was actually a benign watch that caused him problems at an airport Wednesday — and the aftermath was a freaking mess.

The actor-turned-politician was held at the Munich Airport on Wednesday after trying to take an “unregistered” luxury watch through customs … the Terminator didn’t try to fight his way out of this one … instead being spotted calmly hanging with customs agents.

Sources with direct knowledge tell TMZ … customs detained AS for three hours for traveling with a watch he owns — one he might auction tomorrow in Austria.

They say Arnold was never asked to fill out a declaration form, but once he got to customs he was hit with what they describe as an “incompetent shakedown.”

But, the former bodybuilding champ didn’t make a fuss … agreeing to prepay potential taxes on the watch — again, a watch he already owns — in order to get on with his day.

Authorities tried to have Arnold use a credit card machine for an hour … but failure after failure forced the customs agents to take Schwarzenegger to a bank so he could withdraw cash from an ATM.

That should’ve been the end of the story … except the ATM had too low a limit — the taxes ended up costing AS $10,613 — and the bank was closed too! The group headed back to the airport, and another officer brought a new, working credit card machine.

Our sources say the watch will still likely be auctioned off tomorrow for the Schwarzenegger Climate Initiative — a nonprofit dedicated to supporting clean energy efforts worldwide.

A spokesperson for the Munich Main Customs Office says it’s simple, telling a local German outlet they’ve initiated criminal proceedings because the watch is an import staying in the EU — it has to be declared.

As for why it took so long, the spokesperson explained the rest of Arnie’s luggage had to be checked … and that takes time.

An exhausting day at the airport for Arnold … filled with more stress than trying to get a Turbo Man action figure!