The announcement comes two weeks after three people were injured in a shooting at the Northwest Folklife festival at Seattle Center.

"The reason for this order is simple," Nickels said at a news conference with Police Chief Gil Kerlikowske. "Our parks, our community centers and our public events are safer without guns."

The executive order, which does not require City Council approval, will apply to anyone with a concealed-weapon permit.

Violators will be considered trespassers and asked to leave city property, but the city does not have authority to impose fines or jail time.

The order directs city departments to report back to Nickels within 30 days with a plan to implement the policy.

Nickels said the city will start by posting signs in city buildings such as City Hall.

The mayor said he hopes the city will not have to require pat-downs or metal detectors in city buildings, but suggested those measures as a possibility for festival organizers of events such as Bumbershoot at Seattle Center.

Washington state law pre-empts all criminal laws on firearms.

"I would not be surprised if there is a challenge to our authority on this," Nickels said. He said a recent lawsuit involving the city of Sequim may give him authority to ban concealed weapons on city property.

State Sen. Jeanne Kohl Welles, D-Seattle, and state Rep. Eric Pettigrew, D-Seattle, also spoke at the news conference in support of the mayor.

Sharon Pian Chan: 206-464-2958 or schan@seattletimes.com