EXCLUSIVE

Thomas Gibson -- best known for his roles on "Criminal Minds" and "Dharma & Greg" -- was arrested early this morning for suspicion of DUI ... TMZ has learned.

Law enforcement sources tell TMZ ... Gibson was driving in downtown Los Angeles at around 1:00 AM when he attempted to drive through a part of town that had been sectioned off for a half marathon. The race had already gone through that part of town (it started at 9:00 PM) but the area was still blocked off.

We're told Gibson was stopped and told he couldn't drive through ... but he proceeded to anyway.

According to our sources, police eventually stopped Gibson and detected an odor of alcohol. We're told Gibson refused to take a breathalyzer test. He was then arrested on suspicion of misdemeanor DUI and taken into custody at 1:37 AM.