A red alert implies a "very high likelihood" of people developing "heat illness and heat stroke", and calls for "extreme care" for vulnerable people, according to the India Meteorological Department.
India has been grappling with unusually high temperatures this summer, and the weather department has said "heatwave to severe heatwave" conditions were likely to continue in several parts, including the capital, through Wednesday.
Commuters have covered their faces with scarves to protect themselves from the harsh heat. (AP PHOTO)
India declares a heatwave when the maximum temperature of a region is 4.5C to 6.4C higher than usual, while a severe heatwave is declared when the maximum temperature is 6.5C higher than normal or more.
Weather stations in New Delhi's Mungeshpur and Narela neighbourhoods recorded a temperature of 49.9C on Tuesday - a record for the city and 9C above normal.
Delhi's local government also restricted the supply of water because of the heat.
It said water levels in the Yamuna River, the main source, were low.
The city does not have uninterrupted water supply at any time, but the government said neighbourhoods that received water for some hours two times a day would be subject to further restrictions.
Children cooled themselves in a lake amid record temperatures in New Delhi. (AP PHOTO)
"I appeal to all the residents that whether there is a water problem in your area or not, please use water very carefully," Water Minister Atishi, who used only one name, said on Tuesday.
Billions of people across Asia, including India's neighbour Pakistan, have been experiencing a hotter summer - a trend scientists say has been worsened by human-driven climate change.
Three more deaths were attributed to heatstroke on Tuesday in Jaipur in Rajasthan state, local media reported, taking the city's toll to four and that of the state to at least 13.
Rising temperatures also prompted India's polling body to make additional arrangements when Delhi voted in the national elections last week, including deploying paramedics, mist machines, shaded areas and cold water dispensers at polling stations.
The elections conclude on June 1 with counting set to take place on June 4.
