Scores of children were seen running at a marathon in Delhi's Chanakyapuri.
Hundreds of children ran a charity road race through poisonous smog in New Delhi on Thursday, triggering a storm of criticism about the damage to their health.
Delhi and other cities in the north have again been blanketed by haze that hits each winter due to build-up of vehicle fumes, industrial emissions and smoke from agricultural fires in regions around the capital.
The toxic air in the capital has taken pollution levels to almost 20 times World Health Organisation safe limits -- causing authorities to close all schools until Friday, ban construction and impose road rationing for drivers with odd number registrations one day and even the next.
But despite the conditions, children as young as nine competed in a road race through the city organised by a non-profit organisation to mark International Children's Day.
The decision to go ahead with the race caused a storm of social media anger.
"I feel all these organisers need to be sued," said Tamanna Sharma, head of an environmental service consultancy on Twitter. "Running in this air quality is a death sentence."
"This is the most idiotic thing to do," said an Indian Forestry Service officer Parveen Kaswan on Twitter.
Vanshika Rawat, 11, who took part in the race, told AFP "we do face a lot of trouble because of the pollution. Our eyes burn, we have difficult breathing".
"You can see there is so much fog around and there's so much smoke. After the run we felt suddenly so exhausted and had difficulty breathing," added 10-year-old Nitakshi Sharma.
Amod Kanth, head of the Prayas group which organised the race, said it was not possible to call off the event at the last minute.
"The children would have come anyway because they wanted to and this event happens every year. They are here and the weather is good, we don't have any problem," said the activist who campaigns for children and the poor.
Reacting to the children's race, climate policy expert Siddharth Singh said the damage to children's lungs caused by the Delhi pollution is "irreversible".
"They will suffer with this for the rest of their lives," he told AFP.
A 2016 WHO report estimated that 100,000 children aged under 14 die each year in Indian cities because of PM2.5 pollution -- the tiny particles that get into the lungs and bloodstream -- in the street and homes.
But medical experts say the Indian capital must brace for a health emergency affecting all ages.
The start of the four-day Panasonic Open India golf tournament was delayed for five hours because of the poisonous cloud.
Some players wore masks when play eventually started and organisers said the event could be shortened if the pollution remains serious.
Arvind Kumar, head of chest surgery at Sir Ganga Ram Hospital in Delhi, said cases of lung cancer among non-smokers are already sky-rocketing.
"In 1988, patients of lung cancer were mostly known smokers. In 2018, 50 percent cases of lung cancer were non-smokers," he told a conference on air pollution in Delhi.
The city is likely to see "an explosion" of lung cancer cases in coming years, he warned.