SAN FRANCISCO (CBS SF) — A 20-year-old man who was a passenger of a dirt bike died Sunday evening after a crash threw him from the bike onto the eastbound Bay Bridge and he was struck by a pickup truck, California Highway Patrol San Francisco said.

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The accident followed reports Sunday afternoon that several hundred motorcyclists were “running amok” in San Francisco, said CHP SF Public Information Officer Mark Andrews.

Around 5:04 p.m., a group of about 15 bikes splintered from the main group and entered the Bay Bridge heading eastbound, Andrews said, but when the drivers saw CHP patrols set up to monitor for stunts and other sideshow activity, they turned around and started heading the wrong direction.

The splinter group of cyclists exited at the Sterling Street and Fourth Street ramps, traversed downtown San Francisco and re-entered the bridge the wrong way on the Fremont Street onramp, heading eastbound on the upper deck of the westbound Bay Bridge, Andrews said.

Andrews said he attempted to divert the group going against traffic onto the Treasure Island ramp but was unsuccessful, and the cyclists reached the new span of the bridge.

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The man who was later killed crashed his dirt bike near the incline and was picked up as a passenger on another dirt bike, which turned around and began heading westbound on the bridge and then exited and attempted to hide from police near the Treasure Island tunnel, Andrews said.

To flee police, the dirt-bike driver attempted to jump the median wall, and that caused the passenger to be ejected onto the Bay Bridge, where he was hit by the truck and killed. The driver, who was also injured, has been arrested.

Dr. Ada Wells posted video on Twitter showing motorcyclists popping wheelies as they headed towards San Francisco in the afternoon.

Another clip posted on Twitter showed a group on similar vehicles pulling stunts as they rode through downtown Oakland.

Video posted to social media showed a number of motorcyclists weaving through stalled traffic in the Yerba Buena tunnel.

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As a result of the collision, two lanes of eastbound I-80 east of the Treasure Island tunnel were shut down for hours Sunday evening, causing major delays. According to CHP, all lanes were reopened as of 9:10 p.m. Sunday.