PORTLAND, Ore. — The Port of Portland and union leaders who represent its marine terminal security guards reached a tentative agreement in labor negotiations Saturday, averting a strike that could have effectively shut down three terminals.

The last-ditch talks led to a resolution by evening after assurances from the Port of Portland that the security guards would continue in their current duties. Both sides had agreed on most aspects of a contract, but workers wanted an assurance their jobs wouldn’t disappear if terminal operators or carriers wanted to hire their own, lower-cost security personnel.

“This agreement protects good-paying, blue-collar jobs that Portland working families need so badly,” Jerry Hardman, ILWU Local 28 president, said in a statement issued by the union.

Port officials had said companies would be more likely to do business in Portland if they were not handcuffed to job guarantees. Portland’s Terminal 6 is by far the smallest of the West Coast’s six container-shipping ports, and it’s already at a disadvantage with other Pacific ports because it’s about 100 miles from the ocean.

The tentative agreement must go before the union for a vote and the Port Commission for formal review and approval of the contract.

The negotiation came 12 hours before a planned 6 a.m. strike Sunday that could have effectively shut down three terminals, the union said. It was assumed the longshoremen who load and unload ships would have refused to cross the picket lines.

“We are very pleased that an agreement was reached,” Port Executive Director Bill Wyatt said in a statement. “The Port feels its contract proposal was not only fair but generous.”

A day before the agreement, Gov. John Kitzhaber tried to head off a walkout by the guards, telling both sides that he expected a deal Saturday, The Oregonian reported. Contract talks began in June 2011.

The looming strike had led some businesses to use ports on Puget Sound, and shipping lines were evaluating whether to bypass the city. When ships are diverted and cargo rerouted, that adds costs, and potentially harmful delays for imports or exports of perishable or seasonal items.

The drawn-out negotiation over security guard contracts was just one of three separate labor conflicts at the Port of Portland.

The first began this summer when the ILWU and the International Brotherhood of Electrical Workers clashed over which union should have the job of plugging in and unplugging refrigerated shipping containers. The National Labor Relations Board and a federal judge concluded that longshoremen engaged in slowdown tactics during the dispute, causing truck traffic to be backed up for more than a mile.

Though the NLRB eventually concluded the work belongs to the electrical workers, the longshore union continues to fight for the jobs.

Separately, in a standoff that involves several ports, the union contract between longshoremen and companies that operate grain terminals on the Columbia River and Puget Sound expired Sept. 30. The companies have made what they say is their last offer, and are awaiting a response.

A strike or lockout would disrupt trans-Pacific grain shipments, affecting U.S. farmers exporting wheat, corn and soybeans to Asia.