Dense, white fumes of sulfur trioxide gas leaked from a fertilizer and insecticide factory in west Delhi today, sending people in the densely populated older parts of the Indian capital into a panic one year and one day after a gas leak from a Union Carbide plant at Bhopal killed more than 2,000 people.

Around mid-morning, a pipe from a tank of concentrated sulfuric acid at the Sriram Foods and Fertilizers plant ruptured, spilling the acid. The acid, called "basic to the insecticides industry" by Vijay Mehta, senior executive at the plant, was neutralized with lime, a process that triggered a chemical reaction, giving off dense fumes that carried over more than seven square miles in some of the most densely populated areas of New Delhi.

Hundreds of people around the largely commercial areas of old Delhi were affected as the gas settled, causing breathing difficulties and severe irritation to the eyes and throat. At least 642 people were treated at six hospitals in the city.

No casualties were reported by the leak of what Mehta termed "a middle category gas," which is corrosive but "not toxic and causes no adverse long-term ailments."

A doctors at the Lohia hospital seemed to concur, saying patients suffered "localized irritation and mild imflammation of eyes and throat." Those admitted were "being kept under observation."

Three senior executives of the plant responsible for its operation and safety have been arrested, and the plant has been closed until "full inquiries are made," said a senior superintendent of police at the plant.

The factory is owned by one of the larger private industrial firms in India, the Sri Ram group.

One of the victims, Pinki Sehgal, a teacher in a primary school in Chandni Chowk, said at a hospital tonight that her breathlessness and hoarseness had increased in the evening.

"Suddenly this thick fog settled on us, and I felt choking," she said, describing her experience today. "After Bhopal the first thing I thought of was, 'Will I live?' "

During the day, authorities issued repeated appeals for calm and warned people to cover their faces with wet towels. State-run All India Radio said an emergency command was set up at the Health Ministry to monitor the situation.

By evening, the flow of injured persons seeking help at hospitals had tapered off.

The doctor at Lohia hospital said, "It is too close to Bhopal for comfort. Everyone remembers and will want answers, even though this is not so bad."

Thirty tons of gas leaked today. The plant produces 100,000 tons a year.