Hundreds of motorcyclists, many on off-road dirtbikes and ATVs, paraded around San Francisco and nearby cities on Sunday mostly evading capture by authorities. But when two motorcyclists tried a daredevil move on the eastern span of the Bay Bridge, it ended with one of them dead.

It's not clear where the gathering of bikers began, but the huge mob of motorcyclists went marauding around freeways on Sunday, driving in the wrong direction in lanes of the Bay Bridge, and generally causing chaos and traffic. This led to some sideshow activity on the bridge, as ABC 7 reports, that then led to an accident that injured one 20-year-old biker who abandoned his bike on the bridge. That biker then got on the back of someone else's motorcycle to make an escape.

Around 5:20 p.m., on the eastern span just east of Yerba Buena Island where the eastbound and westbound decks of bridge are parallel, the bike with the two riders on it attempted to cross a concrete barrier to evade capture by police — going possibly from the westbound lanes, where bikes were going eastbound in the wrong direction, to the eastbound lanes. As the Chronicle reports, the "bike didn’t make it over, and the cyclist who had become a passenger flew off and was struck by a pickup truck."

Subsequently, traffic backed up and two lanes of the eastbound bridge were closed for several hours.

KTVU and ABC 7 have some video of the traffic and the bike activity on the bridge.

As the Chronicle reports, it was a "splinter group" of motorcyclists who headed onto the bridge just after 5 p.m., and they were met by CHP officers who were anticipating such a move. The accident then occurred about 15 minutes later.

Officer Mark Andrews tells the paper that "several hundred" bikers were involved overall, and it's unclear if any arrests have yet been made.

The motorcyclist who died has not yet been identified.

Sunny Sundays have traditionally been times for illegal sideshow activity around the Bay, not always involving dirtbikes. An attempted Sunday sideshow on the Bay Bridge was thwarted by the CHP five years ago.