Salil Shetty, Amnesty's secretary general, issued his warning as he arrived in Brazil at the start of a week-long visit.

Last week more than 200 Brazilian police swept through Rio de Janeiro's largest slum in a crackdown on drug-related crime, arresting 11 people and seizing an estimated three tonnes of marijuana.

The raid was part of series of efforts which began more than two years intended to clamp down on the deadly violence in Rio's slums - or favelas - ahead of the 2014 World Cup and the 2016 Olympics.

Around a third of Rio de Janerio's population of six million live in the favelas in and around the city.

So far, police operations have been organised in more than 20 favelas, with drug organisations expelled.

"Our worry is that because of the·Olympics now this thing could get scaled up very significantly," Shetty said in an interview with Reuters.

Late last year, officials declared victory after 2,600 paratroopers, marines and elite police, backed up with helicopters and armoured personnel carriers, led a pre-dawn assault on the Vila Cruzeiro and Complexo do Alemao favelas that ended with 37 suspected wrongdoers killed, a score of arrests and the seizure of more than 500 weapons.

Rio de Janeiro plans to build three expressways for buses ahead of the Olympics that will pass through the favelas and Shetty is concerned that the drug problem will be used as a smoke-screen to clear them out of the city.

"Everybody fully understands that some degree of movement might be inevitable when you're undergoing such a major project, but the issue is whether the fair process is being followed," Shetty said.

"These people have been given houses which are 50 km (31 miles) away from their livelihood, or compensations which are really a pittance.

"The communities are not really involved."

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