India's capital Delhi is blanketed in toxic smog every autumn, but the pollution is already so bad that it's drawn protests as authorities tell students to stay home.

The Delhi police have also registered a case under the anti-terror law though they haven't given a reason for it. So, there is no clarity yet on what caused the blast and who, if anyone, is behind it. Authorities say eight people were killed in the blast, while 20 others were injured. An update is expected later today.

A car blew up near the historic Red Fort in India's capital New Delhi, killing at least eight people, police said, adding that the cause remained unclear.

Those responsible for the explosion “will not be spared,” Prime Minister Narendra Modi of India said. The blast killed at least eight people near a subway station at evening rush hour.

At least eight people were killed on Monday in a car explosion near the historic Red Fort in a densely populated area of the Indian capital of New Delhi, a police spokesperson said. It was a rare blast in the heavily guarded city of more than 30 million people.

Delhi's air quality has deteriorated to "severe" levels, which can seriously impact those with existing diseases.

The move follows a weekend protest where police detained dozens of people demanding cleaner air, a rare public demonstration against pollution in the Indian capital.

The blast-like sound near the Radisson Hotel in southwest Delhi's Mahipalpur area caused panic among residents in Delhi