Workers carry sweets at a shop in Ahmadabad, India. Diwali, the festival of lights will be celebrated on Oct. 17 amid concerns in some regions about tainted milk products. (Ajit Solanki/Associated Press)

Doctors in northern India are warning residents to avoid eating dairy-based sweets during a major Hindu festival this weekend after police raided two factories and found them using detergent and animal fat in the manufacture of milk and milk products.

The warning comes as India prepares for Diwali, the festival of lights, on Saturday, when residents are expected to consume and give gifts of sweets, many of which are made from milk products.

Police said 28 people were arrested in raids Wednesday in two towns in Uttar Pradesh state, and about 1,000 litres of synthetic milk were seized.

"Milk products manufactured from urea, caustic soda and animal fat were recovered from two makeshift factories," said police official Brij Lal.

N.C. Khanna, a scientist at the Indian Institute of Toxicology Research, said the synthetic milk is prepared by mixing the organic compound urea with caustic soda, cheap cooking oil, water and a common detergent. The detergent emulsifies and dissolves the oil in water and creates a frothy white solution that looks like milk.

The blend is then added to natural milk before it is sold, he said.

Doctors say the synthetic milk is carcinogenic and urea and caustic soda are harmful to the heart, liver and kidneys.

Caustic soda, found in oven and drain cleaners, is particularly harmful even in small doses and could prove fatal for people suffering from hypertension and heart ailments, according to Dr. Lalit Saxena, a scientist at Biomab Pharmaceuticals Private Ltd, in Goa, India.

"Please do not eat sweets this Diwali. Avoid them as there is no guarantee of its purity," said Dr. D.P. Mishra of the state-run Balrampur Hospital in Lucknow.

Last November six children died and more than 60 fell ill after drinking adulturated milk in a state school in eastern India.

The news comes about 13 months after a Chinese dairy recalled hundreds of tonnes of baby formula in one of country's worst food safety scandals. Milk powder contaminated with the industrial chemical melamine killed at least six babies and sickened almost 300,000 others with painful kidney stones.

With files from The Associated Press