Updated

Heavily armed men halted buses and cars, robbed their passengers and set the vehicles ablaze Wednesday in Rio de Janeiro, continuing a wave of violence that has rattled rich and poor alike in a city Brazil hopes to make a showplace for the 2016 Olympics.

A police spokesman said at least four buses and 10 cars were burned in Rio's poorer northern and western areas overnight. Police invaded slums in those areas overnight and engaged suspected drug gang members in intense shootouts. It was not immediately known if there were any deaths from the fighting.

Security officials say drug gangs have mounted four days of violent attacks across the city to convince authorities to drop a get-tough program that involves having police push gangs out of shantytowns where they have long ruled with impunity.

Gangsters armed with assault rifles and grenades have used cars to block major thoroughfares, then robbed people snarled in the resulting gridlock and set some cars ablaze. At least one man was killed resisting the bandits.

Violence has plagued Rio for decades, but most has been contained within the slums that cling to the hillsides. Now, however, at least a few of the recent attacks have spilled into middle class and wealthier neighborhoods closer to the beach, spreading fears that police are losing control of the city.

"The scary part is that now it's getting close to us. Before the violence was always far away," said Olga Silveira, who was milling around a plaza in the wealthy Ipanema neighborhood where police blew up a large, empty wooden box mistakenly feared to contain explosives. "Now we're feeling it on our flesh. The criminals have discovered the power they have and they want to show it."

Reginaldo Maciel, a police officer patrolling Ipanema, said the suspected bomb around 7 a.m. Traffic was halted for several hours on a few streets around the plaza — causing havoc for morning commuters trying to travel through the neighborhood, whose streets are a traffic choke point..

Burned-out husks of cars littered the sides of roads in several parts of the city and thick plumes of black smoke billowed into the sky as they burned.

Police have responded by deploying riot officers on expressways into the city of 6 million people and sending patrols into more than 20 gang-controlled shantytowns to hunt down gang members they hold responsible for the attacks.

Officers seized drugs, cells phones and various weapons, including a Kalashnikov in the action. Two men died confrontations with police Tuesday, officials said. Eleven have been arrested.

Rio state Public Safety Director Jose Beltrame, in charge of the security forces, said the attackers are trying to disrupt a city campaign to push gangs out of key shantytowns.

"There are groups of criminals who have been installed here for 20, 30 years, and they might not want to give up. But we're not giving up either," he said. "If they keep this up, we'll redouble our efforts. Anyone who gets in our path will be run over."

He said eight high-level drug traffickers are suspected of coordinating the attacks from local prisons and asked they be moved to more distant federal penitentiaries.

Thirteen shantytowns have been pacified over the past two years. The plan is to free 40 — a small fraction of Rio's more than 1,000 slums — of gang control by the time of soccer's 2014 World Cup.

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Associated Press writer Bradley Brooks in Sao Paulo contributed to this report.