Chile declared a health alert at a site where 500,000 pigs, abandoned by a slaughtering company, were dying in large numbers until authorities stepped in Tuesday.

Health Minister Jaime Manalich visited the plant in Freirina, 800 kilometers (500 miles) north of Santiago in the parched Atacama region, and said that without food, water, cleaning or care the pigs had started to die.

The incident began when Freirina residents, angry about foul odor from the swine, held a march Thursday that culminated in riots. Demonstrators set two police cars on fire, and police arrested 23 protesters.

The angry locals closed access roads to the pig slaughtering site, and employees fled the plant.

"Because of the road closures from Thursday onward the animals have stopped receiving food, there is no sanitation, their waste is not being disposed of and we understand there is high mortality among the animals, particularly the young piglets," said Manalich.

Authorities declared a sanitary alert in the area and temporarily closed the plant, owned by the firm Agrosuper.

However by late Tuesday Manalich reached an agreement with protesters to end the road closure and allow Agrosuper workers to enter the plant and feed the swine.

Waste is overflowing at the plant's waste treatment unit, some animals have escaped their pens and others had died, the minister told local media.

The waste overflow could contaminate area drinking water "and could be a grave danger for the population," Manalich said.

Manalich said that authorities agreed to temporarily close down the plant.

Plant officials acknowledged problems in the slaughtering plant ventilation system that increased the foul odors.