RIO DE JANEIRO, March 23 (Reuters) - Powerful home-made explosive devices were found on the roof of a children's nursery school in Rio de Janeiro and police said on Friday they suspect a drug gang used the building as a weapons warehouse.

As about 30 children played next door, officers removed two dozen pipe bombs and enough explosive materials to make dozens more from the building in the giant Mare slum complex, a police officer at the precinct investigating the case said.

"The charges were in a box on the roof. It was very dangerous as they could have been set off by the heat of the sun," he said.

Police who raided the area on Thursday acted on a tip from residents that local drug traffickers had stored something in the building.

Many of Rio's 600-plus slums are dominated by gangs dealing in drugs and arms. These criminal groups clash almost daily between themselves in bloody turf wars or with police.

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