"At 6:30 a.m., Sector Houston-Galveston watchstanders received notification that approximately 11 individuals were suspended from the Fred Hartman Bridge", the release said. "The Houston Ship Channel is closed from Light 102A to Light 104".
The environmental group Greenpeace said it was protesting fossil fuels ahead of a debate in the city by Democratic presidential candidates.
"As we hang here, the Democratic candidates are getting ready for the #DemDebates just a few hours from now here in Houston,” Greenpeace said in a Twitter statement. “We challenge every candidate on stage tonight to promise to hold the fossil fuel industry accountable if they become president. #PeopleVsOil”
The Coast Guard said the closure had already blocked one inbound vessel and two outbound ships.
Greenpeace said 22 "climbers" took part in the protest. Pictures and video posted by the group to Twitter showed them unfurling banners from a bridge across the waterway.
It was unclear whether police might move to arrest the protesters.
Harris County Sheriff Ed Gonzales said via Twitter that no water vessels were being allowed to pass through the area.
"I’m at the Fred Hartman bridge. Two northbound lanes on the bridge are shutdown", he said. "Our priority is the safety of everyone. We will provide updates as they become available".
One of the protesters, Trace Redd, vowed via Twitter to remain suspended from the bridge and continue to block ships from leaving for up to 24 hours.