Anti-driving protest

One Friday we were driving back home from work and were stopped on one of the main roads by a huge group of young people (teens and up) on bikes, rollerblades, skates etc. As a caveat, we don't typically drive to work since taking the light rail is so convenient plus gas is so expensive...
Anyways, our car got blocked at one intersection for a few minutes and we had no idea why. The group rolled in and just stopped in the middle, started playing music and just talking and hanging out.
Eventually, as the kids kept passing by, we realized that it was an anti-driving protest.
We noticed signs and flags with car pictures crossed through.
My camera is pretty slow so I did not catch a picture of the flags, but here is some shots of the group...
Apparently this happens every Friday.
How we know this? Completely unrelated to this day, we were walking one Friday when we were still staying at the Novotel, we were crossing over the pedestrian bridge to go have dinner in the Old Town. We passed a huuuuuuge crowd of youngsters on the bridge, all on bikes, skates, rollerblades (all smoking pot by the way.. haha...). We had no idea what that was about.
Well now we have the answer...