Children put candles on the ground Wednesday during a vigil on the eve of the 25th anniversary of the disaster in Bhopal, India. (Altaf Qadri/Associated Press)

Hundreds of people marched with torches through Bhopal before dawn Thursday to mark the 25th anniversary of the world's worst industrial disaster and demand the cleanup of toxic chemicals they say still contaminate the Indian city's soil and water.

Early on Dec. 3, 1984, a pesticide plant run by Union Carbide spewed about 36 tonnes of deadly methyl isocyanate gas into the city's air, quickly killing about 4,000 people. The lingering effects of the poison raised the death toll to about 15,000 over the next few years, according to government estimates.

Local activists insist the real numbers are almost twice that, and say the company and government have failed to clean up toxic chemicals at the plant, which closed after the accident.

"Down with the government," and "Down with Union Carbide," the protesters chanted Thursday as they marched to the plant in Bhopal, the capital of Madhya Pradesh state.

"Punish the guilty and remove the toxic waste from the plant that still contaminates the soil and groundwater," said Rashid Bi, a victim who joined the march.

Dow Chemical Co. bought Union Carbide in 2001. Dow says the legal case was resolved in 1989 when Union Carbide settled with the Indian government for $470 million US, and that all responsibility for the factory now rests with the government of the state of Madhya Pradesh, which now owns the site.

Gas blamed for birth defects

The government says the gas, also known as MIC, affected at least 500,000 people. Activists say thousands of children born to parents directly exposed to the gas or poisoned by contaminated water are suffering from brain damage, cleft lips, missing palates and twisted limbs.

Skin, vision and breathing disorders also are common, they say.

"The enormity of that tragedy of neglect still gnaws at our collective conscience," Prime Minister Manmohan Singh said in a statement Wednesday.

"I reaffirm our government's commitment to resolving issues of safe drinking water, expeditious cleanup of the site, continuation of medical research, and any other outstanding issues connected with the Bhopal gas tragedy," he said.

However, Babulal Gaur, the state minister for gas relief and rehabilitation, insists there is no current toxic contamination and dismisses assertions that the birth defects are related to the disaster. He says the diseases plaguing children are only a consequence of living in poor slums.

Investigations have found that the accident occurred when water entered a sealed tank containing highly reactive MIC, causing pressure in the tank to rise too high.

Union Carbide Corp., an American chemical company, said the accident was an act of sabotage by a disgruntled employee who was never identified, and not the result of lax safety standards or faulty plant design, as claimed by some activists.

Critics say the plant should not have used MIC, which is cheaper than less-hazardous alternatives, and should not have been located in a highly populated area.