HONOLULU -- Protesters filed a lawsuit on Wednesday to block the U.S. Coast Guard's plans to protect the Hawaii Superferry from demonstrators on Kauai.

People who planned to go back into the water to keep Superferry away from Kauai were facing arrest if they entered a huge Coast Guard safety zone. Their lawyer said he has found both constitutional and clerical errors he will use to block the authorities from enforcing the zone.

"If they do, they'll be activating something that doesn't exist, they'll be acting outside the law," attorney Lanny Sinkin said.

The Coast Guard expanded the security zone to all of Nawiliwili Harbor after floating protestors out maneuvered authorities in small boats.

The public was also banned from a jetty that extends into the harbor, which was a staging area for the protest in and out of the water.

Sinkin said the Coast Guard is legally wrong to use its emergency powers to protect the Superferry.

"The laws that came out of 9/11 are being used now to go after simple protestors, not terrorists, not saboteurs, but simple protestors," Sinkin said.

The Coast Guard said the zone is a legally sound way to balance the needs to facilitate commerce while protecting the ferry and potential protestors.

The Superferry is scheduled to resume Kauai service on Sept. 26.

Gov. Linda Lingle is going to Kauai on Thursday night to warn protestors that they could also face state charges if they face off with the ferry.

A hearing over if and when the ferry can resume Maui service could last into next week.