At least 22 children have died after eating a free lunch feared to contain poisonous chemicals at a school in eastern India, officials said Wednesday, as angry protests erupted over the tragedy.

Many more children remain ill in hospital after consuming lunch cooked at a village primary school in the impoverished state of Bihar, state education minister P. K. Shahi said.

"It is sad but true that (more than) 20 children died after eating their midday meal, which appears to be poisonous," Shahi said.

Deadliest Poisons Known to Man: Photos

The dead children, all aged under 10, were buried near the school on Wednesday morning as angry residents armed with poles and sticks took to the streets in the city of Chhapra.

The mob smashed windows of school buses and other vehicles in Chhapra, the main city of Saran district where the school is located.

"Hundreds of angry people staged a protest in Saran since late Tuesday night, demanding stern action against government officials responsible for this shocking incident," said district government official S.K. Mall.

A preliminary investigation has shown the meal of lentils and rice served to the children on Tuesday may have contained traces of phosphate from insecticide, said local government official Amarjeet Sinha.

25,000 Dead Bees in Target Store Parking Lot: Analysis

The cause of death could be organophosphorous poisoning due to insecticide in the vegetables, he told AFP, adding that doctors are treating victims with atropine.

A more detailed investigation is under way into the tragedy at the state-run school in the village of Masrakh, the official said.

Media reports quoted villagers as saying the use of contaminated, foul-smelling mustard oil for cooking at the school could also have caused the deaths.

"Investigators are examining midday meal samples and samples of victims' vomit. Only the final report of inquiry will reveal the real cause," Sinha said.

News: Caffeinated 'Vomit Drink' Nauseated First City

State chief minister Nitish Kumar has announced compensation of 200,000 rupees ($3,373) for bereaved families.

Free lunches are offered to impoverished students in state-run schools as part of government welfare measures in many of India's 29 states.

Bihar is one of the country's poorest and most densely-populated states.

Educators see the midday meal scheme as a way to increase school attendance. But children often suffer from food poisoning due to poor hygiene in kitchens and occasionally sub-standard food.

More than 130 students were taken to hospital in the western city of Pune last year after eating lunch at school, the Times of India reported.

A probe revealed that the food served to them was contaminated with E. coli bacteria, strains of which can cause food poisoning.

Food prices have soared in India over the last six years, causing increased hardship for the 455 million people estimated by the World Bank to live below the poverty line.