SAN JUAN, Puerto Rico--Sabotage felled power pylons and feeder lines and blacked out much of Puerto Rico beginning at daybreak yesterday, utility officials said. They said power outages would likely continue until today.

There was suspicion the sabotage was related to the bitter 30-month contract dispute between the utility workers' union and the government-run power authority on the island, a U.S. commonwealth of 3.6 million people.

The union denied responsibility.

The Puerto Rico Electric Power Authority said about 500,000 households and businesses throughout the Caribbean island, or about half its customers, were affected for periods of up to three hours.

The authority said it was cutting power selectively for two-to three-hour periods because of reduced capacity, and would continue doing so at least until tomorrow while repairs were made.

Huge traffic tieups were reported at the 6 p.m. rush hour in San Juan, the capital and home to about 1 million people.

The power authority's executive director, Jose del Valle, said two transmission towers and two poles supporting feeder lines were damaged and that inspectors determined the equipment had been vandalized.

Del Valle would not speculate on who was responsible. He said one pylon was knocked down at Aguas Buenas and the other near Bayamon, both in the greater San Juan area. Elsewhere, two feeder line supports were knocked flat.