Look into the 1284x key - it's used by the majority of police cars in the US , and you can easily find it on ebay for super cheap. Once you get one, you can get it copied at a hardware store and hand them out to comrades before protests.
Here's a talk discussing things that are commonly keyed, it's very good and definitely worth your time to watch https://www.youtube.com/watch?v=a9b9IYqsb_U (I was planning to make this post today but u/alfrednobel already posted the same talk lol, guess we had the same idea)
Other keys to look into would be the ones to doorking systems for apartment buildings - those boxes that let you buzz in to go inside. They have common maintenance keys, and with that key and a piece of wire you can let yourself into the building if you need to hide out for a while. These are the 16120 and 222343 keys. Again, watch the talk, it goes into this.
Stay safe comrades!
I completely forgot about that part of the talk, lmao
I wouldn't see that being super useful at a protest, especially since you'd need to carry a large ring of keys to make sure you had the right one for any equipment you come across, but driving a backhoe or bulldozer through a line of cops sounds fucking hilarious.
I figure that if you know the general area you can check out construction sites beforehand. If you are going to take one you might want to learn a bit about how to drive them first as well. having one key on a key ring will get overlooked and leave you the option. Most of this is stuff that would only be needed if things get a bit hotter than they are now though.