Unless they were also blocking your way in the other direction, I'd think it would be difficult to make the case that you were being detained.
The closest I've ever come to being in this position was when I lived in a small town in northern California. I got up one Saturday morning to drive to the nearby Naval Station and the road outside my house was literally solid curb to curb with bicycles - hundreds of them. They were preparing to start off on some kind of mass ride for something-or-other, and apparently the starting line was right outside my house on this little side street we lived on. I stood beside my car and shouted to anyone who'd listen that I needed to get to work, but they all pretended not to hear me. So I got in my car, started it up and began to drive very slowly right into the crowd. They moved.