Prime Minister Narendra Modi has called the explosion on Monday evening a “conspiracy”, and has vowed that those responsible will face justice.

Police are yet to give exact details of what caused the intense explosion near the historic Red Fort in the crowded Old Delhi quarter of the city, one of India’s most well-known landmarks, and the site of the annual prime minister’s Independence Day speech.

It was the most significant security incident since April 22, when 26 mainly Hindu civilians were killed at the tourist site of Pahalgam in Indian-administered Kashmir, triggering clashes with Pakistan.

“Twelve people have died and more than 30 are injured,” Ritu Saxena, the chief medical officer of Delhi’s LNJP hospital, told AFP.

India’s National Investigation Agency is leading the probe into the explosion, which came hours after police said they had arrested a gang and seized explosive materials and assault rifles.

Police said the men were linked with Jaish-e-Mohammed, a Pakistan-based Islamist group, and Ansar Ghazwat-ul-Hind, a Kashmir offshoot of jihadist group Al-Qaeda.

India lists both groups as terrorist organisations.

Home minister Amit Shah, after chairing security talks following Monday’s blast, said he had instructed officials “to hunt down each and every culprit behind this incident”.