Protesters, reportedly in a car caravan, shut down the upper deck of the Bay Bridge early Sunday evening as part of a Black Lives Matter protest, creating a backup that extended past Treasure Island and into the East Bay.

It's unclear how many demonstrators were involved in the protest, but traffic was at a standstill for a considerable amount of time after 5 p.m. And at one point an ambulance was trying to get to a protester who was suffering an asthma attack, according to a Twitter a post.

KRON 4 reports that the standstill was the result of a 50-car caravan that slowed down traffic in a coordinated effort.

The Mercury News reports that demonstrators exited cars and began stenciling and spray-painting "Black Lives Matter" on the roadway of the bridge.

ABC 7 reports that around 20 arrests were made in the demonstration, but that number has not been confirmed.

According to the Mercury News, as many as 27 people were detained in the demonstration by the SFPD and San Francisco Sheriff's Department. And no one was injured.

This post was updated throughout.