HOUSTON (AP) — Eleven protesters have been arrested after they rappelled from a busy highway bridge over the Houston Ship Channel, closing the vital artery for hours.

The Coast Guard said the closure came after the protesters were spotted Thursday afternoon dangling from the Fred Hartman Bridge near Baytown, Texas.

The demonstrators were Greenpeace USA activists protesting the use of fossil fuels. They suspended themselves from a bridge ahead of Thursday night's Democratic presidential debate in Houston.

Firefighters rappelled to the protesters Thursday evening and lowered them to boats below. Harris County Sheriff Ed Gonzalez says 12 activists serving as spotters on the bridge also were arrested.

The Port of Houston, located along the ship channel, is home to the largest petrochemical complex in the U.S.