Protesters are demanding action to clean up Bhopal site.
People in the Indian city of Bhopal have been marking 25 years since a leak at a gas plant killed thousands and left many more seriously ill.
Activists and survivors marched through the city, chanting slogans against the government and Union Carbide - the US firm that owned the plant at the time.
The incident was the worst industrial disaster in history.
Forty tonnes of a toxin called methyl isocyanate leaked from the factory and settled over slums on 3 December 1984.
Campaigners say at least 15,000 were killed within days and that the effects of the gas continue to this day.
The site of the former pesticide plant is now abandoned. The state government of Madhya Pradesh took it over in 1998, but environmentalists say poison is still found there.
Residents of the city have held a week of commemoration events in the lead-up to the anniversary.
Early on Thursday, protesters marched to the site chanting: "Down with the government", and "Down with Union Carbide".
BHOPAL'S DEATH TOLL
Initial deaths (3-6 December): more than 3,000 - official toll
Unofficial initial toll: 7,000-8,000
Total deaths to date: over 15,000
Number affected: Nearly 600,000
Compensation: Union Carbide pays $470m in 1989
Source: Indian Supreme Court, Madhya Pradesh government, Indian Council of Medical Research
One victim who joined the march told Reuters news agency: "Punish the guilty and remove the toxic waste from the plant that still contaminates the soil and groundwater."
The BBC took a sample of water from a hand pump in constant use just north of the plant and had it tested in the UK.
It contained nearly 1,000 times the World Health Organization's recommended maximum amount of carbon tetrachloride, a pollutant known to cause cancer and liver damage.
The state government of Madhya Pradesh is now responsible for the site and earlier this week Chief Minister Shivraj Singh Chouhan told the BBC that the water supply around the plant was safe.
On Wednesday, Indian Prime Minister Manmohan Singh said: "The enormity of that tragedy of neglect still gnaws at our collective conscience."
He reaffirmed his government's "commitment to resolving issues of safe drinking water, expeditious clean-up of the site, continuation of medical research, and any other outstanding issues connected with the Bhopal gas tragedy".
Campaigners say Bhopal has an unusually high incidence of children with birth defects and growth deficiency, as well as cancers, diabetes and other chronic illnesses.
This is seen not only among survivors of the gas leak but among generations born much later, they say.
No-one has ever stood trial over what happened at Bhopal.
Twenty years ago, Union Carbide paid $470m (£282m) in compensation to the Indian government.
Dow Chemicals, which bought the company in 1999, says this settlement resolved all existing and future claims against the company.