The 13 Greenpeace protesters, who rappelled down from the bridge over the Willamette River on Wednesday, are hoping to shorten Shell's Arctic drilling season by stopping the Fennica icebreaker, which is carrying emergency equipment that would cap any blown-out well.
Shell is not allowed to start drilling without it and the drilling season ends in October, when sea ice forms.
Greenpeace says Arctic drilling could be damaging to populations of whales, polar bears and walrus in the event of an oil spill.
Greenpeace is the latest group to stage demonstrations over the past three months in a bid to block Shell vessels from heading north, following similar demonstrations in Seattle and other Washington state ports.
The Fennica stalled once it neared the St. John's bridge on the Willamette, which leads to the Pacific Ocean. Protesters lowered themselves with ropes to prevent the ship from passing under the span, forcing the vessel back to port.
The could be seen dangling from ropes with large red and yellow banners, at times chanting "Shell No".
Meanwhile, waterborne U.S. Coast Guard backed by sheriff's deputies were using loud speakers to order dozens of kayakers off the water, though the sympathetic paddlers ignored them.
Police closed the bridge to road traffic and scores of officers assembled at the bridge, possibly gearing up to arrest the dangling activists.
Onlookers sympathetic to the protests gathered in a nearby park and occasionally cheered in support.
"When that ship turned, that was history," said kayaker Michael Foster.
Shell hopes to return to the Arctic for the first time since 2012, when it experienced a series of mishaps including the grounding of an oil rig. There was no word on the Fennica's departure plans.
Company spokesman Curtis Smith said Shell respects the rights of individuals to protest but that "the staging of protesters in Portland was not safe nor was it lawful."
As the protests stretched into the afternoon, a U.S. judge in Alaska held Greenpeace in contempt and ordered it to pay fines of $2,500 per hour if the protest continues, with fines increasing daily to a rolling $10,000-per-hour after Aug. 2.
Greenpeace USA Executive Director Annie Leonard said in a statement it was evaluating its options but that its protesters would stay in place for the time being.
"While we respect the courts, we also respect the increasingly urgent science that tells us Arctic oil needs to stay underground," Leonard said.
Additional reporting by Timothy Gardner in Washington and Eric M. Johnson in Seattle; Editing by Eric M. Johnson and Sandra Maler.
