NEW DELHI — More than 25 years after a plume of fatal toxic gas from an American-owned chemical plant wafted over the slumbering city of Bhopal, eight former executives of the company’s Indian subsidiary — including one who has since died — were convicted Monday of negligence. The seven surviving defendants were sentenced to two years in prison and fined 100,000 rupees, or $2,100.

They were the first criminal convictions from the leak at the Union Carbide chemical plant in Bhopal, a central Indian city. The leak killed 3,000 people almost instantly, and thousands more died later from the aftereffects of the toxic gas, an ingredient in pesticides the plant produced.

Victims groups and activists, who had sought more serious charges, criticized the verdict. Death by negligence is most frequently used in deaths involving car accidents, they said. It carries a maximum two-year sentence.