The officers were searching a piece of forest by a farm in Resende, northwest of Rio de Janeiro, yesterday when "they found a box," the state police force said in a statement today.
"On opening it, they encountered a swarm of bees and were attacked."
Seven officers required hospitalisation, two of them being quickly released, the statement said. Of the five still in hospital, one was in intensive care.
Globo News television reported that one of the victims was stung 300 times.