At least 22 children have died and dozens more have fallen ill after eating lunch at a school in the eastern Indian state of Bihar.
The poisoning occurred in the village of Dharmasati Gandaman, 80km (50 miles) north of the state capital, Patna.
The free Mid-Day Meal Scheme aims to tackle hunger and boost attendance in schools, but suffers from poor hygiene.
Angry parents joined protests against the deaths, setting at least four police vehicles on fire.
An inquiry has begun and 200,000 rupees ($3,370) in compensation offered to the families of each of the dead.
Twenty-eight sick children were taken to hospitals in the nearby town of Chhapra after the incident and later were moved to Patna.
A total of 47 students of the primary school fell sick on Tuesday after eating the free lunch.
There are fears the number of dead could rise as some of the children, all below the age of 12, are critically ill.
PK Shahi, the state's education minister, told BBC, "The doctors who have attended are of the tentative opinion that the smell coming out of the bodies of the children suggests that the food contained organo-phosphorus, which is a poisonous substance.
"Now the investigators have to find out whether organo-phosphorus was accidental or there was some deliberate mischief."
Mr Shahi acknowledged "that food is not being checked before it is being served".
He added that "the scale at which the operation is being carried out, serving food to 20 million children every day and that too in remotest village schools, checking food before it is served - that itself is a challenge".