SAO PAULO, Brazil -- Environmental activist group Greenpeace said on Monday some of its members prevented a cargo ship carrying genetically modified Argentine soybeans from topping off its load in Paranagua, Brazil's main grain port, which has banned GMO soybeans.

"Greenpeace will not permit GMOs coming from other parts of Brazil or from the world to contaminate the unique GMO-free port of the country," Gabriela Vuolo of Greenpeace's movement against GMOs said.

A spokeswoman for the port of Paranagua was not immediately available for comment.

The state of Parana, home of Brazil's main grain port and the country's largest producer of grain, banned GMO soybeans at the port in October 2003. Although they are legally grown in nearly all states in Brazil, GMO soybeans are estimated to make up well under half of Brazil's crop.

Virtually all of the soybeans grown in Argentina are GMO.

Greenpeace said that at 10:00 a.m. local time some of its members it had blocked the Global Wind, carrying 30,000 tonnes of Argentine soybeans, from entering the port of Paranagua, presumably to take on more soybeans from Brazil, a common practice called topping off.

Some Argentine ports are too shallow for ships to load fully. Brazil is winding down its soybean harvest and would have no reason to import soybeans.

"The economic advantages of being the largest supplier of GMO-free soybeans in the world could be lost if there is not control to avoid contamination," Vuolo said. The blockage is part of a Greenpeace campaign titled "A Better Brazil without GMOs."

Local agricultural analysts, however, say Paranagua's effort to ban GMO soybeans in search of better prices will likely backfire and that the port and exporters are at risk of losing money as well as disrupting Brazil's soy exports.

March soybean exports from Paranagua plunged 62 percent to 247,300 tonnes compared with the same month last year, while soybean exports in all other large ports grew significantly during the month, Brazil's Trade Ministry said in April.

Paranagua has been plagued with problems this year including rains that interrupted ship loading, system failures, strikes by the port's exporters, truckers, health and farm inspectors, truck lines stretching over 100 kilometers (63 miles) and a ship waiting time of up to 22 days.

Exporters have reported moving business to other ports to avoid delays at Paranagua.

Latin America's largest shipping port of Santos benefited most from the flight of soybean business from Paranagua.

Santos shipped 792,300 tonnes of soybeans in March compared with 477,200 tonnes in the same month last year. The port of Vitoria shipped 245,300 tonnes versus 97,400 tonnes last March. The southern port of Rio Grande shipped 120,500 tonnes of soybeans in March, up from zero a year earlier.

Brazil, now nearing the end of its soybean harvest, is the world's No. 2 soy producer and exporter after the United States.