Portland, Ore. — Authorities moved in Thursday and forced protesters in kayaks from a river in Portland, Ore., where the demonstrators were trying to stop a Royal Dutch Shell icebreaker from leaving dry dock and joining an Arctic oil drilling operation.

Police also tried to lower protesters who were dangling from a bridge into the water below.

Portland police spokesman Sgt. Pete Simpson said safety was the main priority as authorities tried to move protesters away from the area. Police and Coast Guard officers were joined by firefighters and a rope-rescue team.

"This is, obviously, a very unique situation," he said.

Greenpeace spokeswoman Cassady Sharp said the people on the St. Johns Bridge over the Willamette River would leave peacefully if they thought they were in danger.

"We'll just have to see how their forced removal plays out right now," Sharp said. "Right now, it's definitely kind of a holding pattern."

The move by authorities came hours after a federal judge in Alaska ordered Greenpeace USA to pay a fine of $2,500 for every hour that other protesters dangle from a bridge over the river to block the ship.

The bridge is at a key location on the icebreaker Fennica's route from Portland to the Arctic. The ship's journey will take it beneath the bridge, down the Willamette to the Columbia River which leads to the Pacific Ocean.

The icebreaker is part of Shell's exploration and spill-response plan off Alaska's northwest coast. It protects Shell's fleet from ice and carries equipment that can stop the flow of oil that gushes from wells.

In May, U.S. District Court Judge Sharon Gleason granted Shell's request that activists protesting its Arctic drilling plans be ordered to stay away from company vessels and beyond buffer zones.

The Fennica tried to leave dry dock earlier Thursday but retreated when activists dangling from the bridge refused to leave and to let the vessel pass.

The Fennica arrived in Portland for repairs last week. It was damaged earlier this month in the Aleutian Islands when it struck an underwater obstruction, tearing a gash in its hull.

Environmentalists hoped to delay the ship long enough for winter weather to prevent Shell from drilling until 2016. By that time, they hope the Obama administration has a change of heart on the issue.

At the court hearing in Anchorage, Gleason said the hourly fine against Greenpeace would increase over the next few days unless the blockade is lifted. It would jump to $5,000 an hour on Friday, $7,500 an hour on Saturday, and $10,000 an hour on Sunday.

One of the kayak protesters, Leah Rothlein, borrowed her mother's kayak and headed onto the river.

"It's pretty cool," the 26-year-old said. "I was in the water for four hours."

Joling reported from Anchorage, Alaska. Associated Press writer Terrence Petty contributed to this report.