SEATTLE (Reuters) - Protesters who vowed to shut down global trade talks got their wish on Monday as a security scare forced evacuation of the Seattle convention center where officials from 135 countries were scheduled to meet.

Armed police teams with bomb-sniffing dogs carried out a painstaking sweep of the cavernous facility that lasted 5-1/2 hours before it was reopened. Helicopters circled overhead, and at one point two fire trucks arrived with sirens blaring.

Illustrating how nervous authorities are about attacks on the WTO meeting, which starts officially on Tuesday, police said the shutdown was in reaction only to a ``potential security breach.''

``We had reason to believe someone may have entered the building,'' said Clem Benton, a Seattle police spokesman, giving no further details. No group immediately claimed responsibility for the scare, although protesters rejoiced at the disruption.

The convention center was evacuated shortly before 5 a.m. (1300 GMT) when a police officer spotted something suspicious. It was reopened at about 10:30 a.m. (1830 GMT).

``There was nothing found inside, persons or packages,'' Benton said. Delegates and journalists lined up for about 300 yards (meters) to get into the building once it was reopened.

The trade meetings, scheduled to end Dec. 3, are aimed at launching a new round of negotiations to reduce tariffs and other trade barriers in sectors ranging from agriculture and construction to entertainment, telecommunications and electronic commerce.

Opponents say freer trade benefits big business at the expense of workers, communities and the environment.

Hundreds of journalists, locked out of their media center and the meetings, milled about on sidewalks, providing a ready audience for protest groups that rushed to fill the news vacuum.

Police on foot and on horses faced off against dozens of protesters, some dressed as turtles -- representing a species the protesters say are endangered by expanding trade -- and others banging on drums and chanting.

The security concerns delayed a preliminary meeting at which world labor, environmental and citizen groups were to be given a chance to publicly air their grievances before WTO officials including Director-General Mike Moore.

Dozens of delegates who had planned side meetings in the convention center were left scrambling for alternative venues, and some held impromptu news conferences on the street or in a nearby hotel.

``It is an unfortunate start, but I think people who are here determined to make a success of this week,'' said David Byrne, European Union health commissioner.

Protesters, who were expected to mass in the streets by the thousands on Tuesday, the first day of official trade talks, said they were encouraged by the problems at the heavily guarded convention center.

``I'm feeling much more optimistic,'' said George Hylkema, who traveled from his home in Balboa, Calif., to protest the WTO, which many see as favoring the interests of business over environmental concerns and labor rights.

``There's a huge groundswell against the WTO,'' Hylkema said.

In the weeks before the meeting, diplomats warned that heated disputes between countries could scuttle the round before it even gets started. But organizers were confident that trade ministers would agree by Friday on an agenda launching the new trade round, however narrow it may be.

But reaching a consensus could prove a monumental task, as agriculture -- a highly protected sector that stirs passions around the world -- is a difficult issue.

The United States and major agricultural produce-exporting nations want the European Union to scrap their farm export subsidies, which account for 85 percent of the world total.

But the EU, backed by Japan, South Korea, Switzerland and Norway, has refused to give ground, infuriating U.S. farm groups and their allies in the U.S. Congress.