If there were grounds for his arrest, why did it have to take place in public and at this specific venue? Does he only come our of hiding and seclusion to make public speeches? This appears to be a public show-boating of force meant to embarrass, bring fear, and intimidate the masses of sheeple, and to provoke a reaction so certain groups may over-react and get violent. I would call it provocateuring without the involvement of a provocateur. I would also guess that there was great dissapointment from the government perspective that all in attendance remained calm and that no one was provoked into violence.
Isayre, you can't really arrest someone for doing something before he has except perhaps a plot that could kill or injure. If he was carrying in a city, place or area where not permitted he's breaking the law, I haven't heard but highly doubt that he would have been issued a permit, another law, failing to obey a lawful order or resisting in any way (even verbal) can be considered resisting arrest. A real stupid thing to do at that place & this time it could have turned ugly fast, glad it didn't for gun owners he did much more harm than good. The man put a lot of people in a bad position.
From the stuff I've read supposedly the resisting/obstructing part occurred when he locked arms with someone smoking weed that the cops were attempting to arrest. I haven't yet seen a video that shows that part (in tight quarters when the crowd pulled together).