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.