A demonstration of more than 100,000 Spaniards, led by sick people of all ages, wound slowly through the capital last night, moving marchers and onlookers alike to tears. The protest was called by left-wing opposition parties to highlight a national health scandal involving poisonous cooking oil that has claimed more than 140 lives since May.

An estimated 15,000 have been affected and three more deaths were announced by Health Department authorities today, bringing the official death toll to 143. The Socialist opposition's figure is 151. The death list could reach four figures, according to officials as no antidote has yet been found for the toxin. Over 1,000 are hospitalized.

The slogan "government assassins" chanted by the demonstrators underscored wide public indignation that inefficient consumer protection made possible the fraudulent sale of huge quantities of highly poisonous rapeseed oil. Officials now believe a total of 660 tons of rapeseed oil which had been adulterated with chemicals for industrial use were reprocessed and sold as cut-rate olive oil.

Despite intensive research by Health Department officials, collaboration from the U.S. Centers for Disease Control, from European experts and from the World Health Organization, there is still no known cure for a gradual process of muscular paralysis that strikes down the victims.

"So far, all the progress reports that have reached me have not provided any basis for optimism," said Dr. Luis Valenciano, Spain's director general of health. The sick in the demonstration included children crippled in wheelchairs and others very thin and moving with difficulty.

The turnout for the demonstration, which was organized by the Socialist and Communist parties and their corresponding labor organizations, indicated that the handling of the health scandal could be a political time bomb for the government.

Last month the government defeated an opposition motion in congress to reprimand five ministers for negligence. Had it passed, the Cabinet would have fallen.

The demonstrators' anger spilled over into a near riot at the end when leftist political activists formed barricades out of cars and street benches and clashed briefly with riot police.

In response to the scandal, the government has since the summer created a new post of director general of consumer affairs, appointed an all-party parliamentary commission to investigate the circumstances surrounding the mass poisoning and compensated relatives of the victims with up to $30,000. Police have arrested some 30 minor businessmen and street vendors in connection with the processing of the toxic oil.

Critics say the distribution of fraudulent olive oil has long been a practice in Spain and has reflected a general laxity over consumer affairs. What has made the present scandal a large-scale killer is the presence of unknown agents, or the reaction among them, that were used to reconvert the adulterated rapeseed oil for cooking use.

Specimen samples have revealed the presence of azobenzene, aniline and anilide oils. But the toxic presence continues to baffle the scientists. Treatment by antibiotics, cortisone and vitamin E in massive doses has failed to make a lasting impact on the victims.

Under highly publicized campaigns, consumers who had bought the poison from street vendors were able to swap their cans for pure olive oil. This has largely convinced authorities that all the adulterated rapeseed is now accounted for. What is less easy to round up is an estimated large amount of food preserves, some homemade, that may carry the adulterated oil.

Added to somber predictions by experts of continuing deaths is the fear of the effects the poisoning might have children born to affected women. The damage has been found to spread to kidneys, lungs, spleen and the brain. One of the calls during the demonstration was for state abortions for all affected pregnant women. Abortions are illegal and penalized in largely Catholic Spain.