Taking a page from ISIS, fascists, citizens, and cops have increasingly been using their vehicles as weapons to run people over in crowds.

Introducing, rebar caltrops.
A brief DIY thread.

You will need:
rebar (free in most construction sites)
bench vise
angle grinder (with a disc for cutting metal, important)
arc welder
(These last 3 are a must have for any radical community anyway for your standard metal working needs)

-With the angle grinder and bench vise, cut the rebar into lengths of approximately 4 inches, cutting at 45°! (to leave pointed ends)
-W/ vise and wrench, bend each length of rebar 90°
-Place 2 pieces of bent rebar together (see photo)
-Weld
You don't even have to be a good...

...welder. It doesn't have to look pretty.
Important! Let people in the area know where they've been placed, and if possible, pick them up when you move on. Though they're far more effective against tires than against feet, there's still a danger of people stepping on them.

Thicker rebar and shallower cuts resulting in duller pieces will decrease danger to feet w/o making them less effective against tires, because a tire has way more ppi2 than your foot.
Stay safe, stay strong!

There have been some inaccurate responses to this thread.
This is for protecting demos and pickets, NOT for randomly throwing in the highway.
Rebar definitely does the job, hollow spikes not needed. This is widely used.
Nails aren't always worth it. Easier to bend, you don't...

have to cut them, but cutting takes 5 seconds, they're harder to see, and they're definitely going through the sole of a shoe, which you don't want.

Since this thread got big, it's attracted a lot of trash.
1. Misinformation, either intentional or from ignorant folks trying to flex (talk tools and all the bros come out).
DO NOT TRUST info from ppl you don't know. Including me. If it's just a rando you have no reference for...

..why would you assume they're giving you solid info?
2. Obfuscation. Like any other tool, this one is good for some problems and not for others. It doesn't replace a barricade, but it is more mobile & can supplement a barricade, can be deployed in a second if a car is revving

at the back of a moving crowd, etc. You don't want to put this on a street the crowd will have to cross, and yes, warn folks in the crowd w/ wheelchairs.
3. Posturing. This is definitely doing the cops' work, encouraging people to proclaim their credentials, talk about their...

experience, create a culture of bluster and bragging. It often starts with comments like "I didn't see you in the streets". Anyone creating hierarchies of coolness is doing the cops' work. We need everyone, everywhere, doing everything. Talking tactics is risky and should only...

be done hypothetically.