WASHINGTON (AP) _ Bolivian police, aided by U.S. helicopters and Army troops, have destroyed six cocaine laboratories, and Bolivian farmers are showing new interest in switching to legal crops as a result, a top U.S. official says.

Ambassador Edward M. Rowell told a news conference Tuesday that the six laboratories - all burned to the ground by Bolivian police - had a potential production capacity of 5 to 5 1/2 tons a week.

Rowell said the operation has also destroyed 289 other types of facilities involved in cocaine production.

He said the operation is having the intended side effect of forcing down the value of coca leaves, thus prompting farmers to consider switching to alternate crops, such as citrus fruits, rice and beans.

''We're seeing the first signs of the kind of reaction we were hoping this whole operation would produce,'' Rowell said.

The operation is continuing with its complement of six U.S. Black Hawk helicopters and about 170 U.S. soldiers, Rowell said, adding that the program may someday be run entirely by Bolivians. The U.S. troops assist in transporting the Bolivian police and are not involved in raids on the cocaine installations, he said.

Critics have argued that the operation was ineffective because disclosures in the Bolivian press before the raids got underway gave traffickers time to move processing facilities and flee. Also, the Bolivian government had said in advance it would not target production of the coca leaves.

In an interview with the Associated Press in La Paz, President Victor Paz Estenssoro said, ''We must act decisively to confront the cocaine mafia because otherwise they could take over the government, even through democratic means.''

Rowell said Paz Estenssoro's government does not appear to have been shaken despite the political risk he ran in inviting a foreign military presence into Bolivia without prior consultation with the Bolivian Congress.

The cocaine industry has poured an estimated $600 million into Bolivia every year compared to around $500 million brought in by legal exports, Paz Estenssoro said.

According to Rowell, Bolivia is planning to send a delegation to Washington next week to discuss the possibility of receiving additional U.S. assistance to compensate for income lost because of the program.

Rowell refused comment on how much the United States would be willing to offer. Bolivia, which is receiving $60 million in assistance during this fiscal year, mostly in food aid, is asking for $100 million.

Almost all of the cocaine consumed in the United States is provided by Bolivia, Peru, Colombia, Ecuador and Brazil. Rowell said the estimate of Bolivian-supplied cocaine ranges from 20 to 50 per cent of American consumption.