The arresting officers were not in plain clothes, not that it would signify anything if they were. Their clothes were marked "Police" in large letters, and some had DHS patches. As for the unmarked cars, why tip off the thugs and give them a chance to attack the cars? Unmarked vehicles and officers in plain clothes are all normal police usage.
The Feds gave the Mayor of Portland well over a month to clean up, and finally acted. Frankly, I hope the Mayor of Portland tries to do more than just whiiiii-i-i-i-i-i-i-i-iiiiiiine!