RIO DE JANEIRO — More than 3,000 police and soldiers backed by armored personnel carriers raced into Brazil’s biggest slum before dawn Sunday, quickly gaining control of a shantytown ruled for decades by a heavily armed drug gang.

The takeover of the Rocinha neighborhood was the most ambitious operation yet in an effort to increase security before Rio hosts the final matches of the 2014 World Cup and the 2016 Olympics.

The action in Rocinha is part of a campaign to drive the drug gangs out of the city’s slums, where traffickers often ruled unchallenged. The city of Rio de Janeiro has more than 1,000 shantytowns where about one-third of its 6 million people live.