Depends on the sort of plastic. But with most plastics... yes they are. Some plastics are not toxic or not very toxic when burned. But most are. Several can cause cancer. Others will poison you. Just try not to breathe any of those fumes.
When I was little my cousins and I used to burn plastics and pick them up high and pretend the falling flames was napalm or bombs and went around burning fire ants or other bugs. I inhaled so much of that poison.