Indian police are investigating a deadly car blast in the capital under a law used to fight “terrorism”, an officer said on Tuesday, as Prime Minister Narendra Modi vowed to bring to justice all those responsible.

The explosion near the historic Red Fort on Monday evening killed at least eight people and injured 20. It was the first such blast in the heavily guarded city of more than 30 million since 2011.

"Today, I have come to Bhutan with a very heavy heart," Modi said in Thimphu, the capital of the neighbouring Himalayan nation, as he arrived on Tuesday for a scheduled visit.

"The horrific incident that happened in Delhi last evening has deeply disturbed everyone," Modi told a public meeting.

"Our agencies will get to the very bottom of this conspiracy. The conspirators behind this will not be spared. All those responsible will be brought to justice."

ANTI-TERROR LAW

Deputy Commissioner of Police Raja Banthia said Delhi police had registered a case under the anti-terrorism law as well as the explosives act and other criminal laws.

The law, called the Unlawful Activities (Prevention) Act, is India's main anti-terrorism law. It is used to investigate and prosecute acts related to "terrorism" and activities that threaten the sovereignty and integrity of the country.

"Investigation is at a preliminary stage and any comment on it will be too premature," Banthia told reporters.

Near the site of the blast in the city's old quarters, a busy market and tourist area, most shops that shut soon after the explosion were yet to open in the early hours of Tuesday.

Forensic experts were seen scouring the site of the blast, which has been sealed since Monday night and enclosed in white cloth barriers.

Delhi Metro said the Red Fort station had been shut for security reasons.

A map of the area around Red Fort, a historic landmark in Delhi. The car blast is marked with a red circle at the intersection of Chandni Chowk and Netaji Subhash Marg, near Gate No. 1 of the Red Fort Metro Station.

A map of the area around Red Fort, a historic landmark in Delhi. The car blast is marked with a red circle at the intersection of Chandni Chowk and Netaji Subhash Marg, near Gate No. 1 of the Red Fort Metro Station.

Police said a slow-moving car which stopped at a traffic signal exploded just before 7 pm (1330 GMT). Nearby vehicles were also badly damaged.

The explosion left behind mangled bodies and the wreckage of several cars on a congested street near a metro station in the old quarter of Delhi.

There was no immediate information on the occupants of the car, who were presumed to have been killed. Police said they were tracing the owner of the car.

Defence Minister Rajnath Singh said agencies were carrying out a swift, thorough investigation with the findings to be made public soon.

Relatives of the victims gathered outside the nearby Lok Nayak hospital to identify the bodies of their loved ones.

"We at least know that my cousin is here, whether he is injured or not or the extent of his injury, we don't know anything," said a distressed relative who did not want to be named.

MODI IN BHUTAN

The Red Fort, popularly known as Lal Qila, is a sprawling, 17th-century Mughal-era edifice melding Persian and Indian architectural styles, thronged by tourists year-round.

The prime minister also addresses the nation from the fort's ramparts every year on August 15, India's independence day.

Modi is in Bhutan for the 70th birthday celebrations of its fourth king, Jigme Singye Wangchuck.

In April, Modi cut short a visit to Saudi Arabia and returned home after 26 men were killed in an attack on Hindu tourists in the Jammu and Kashmir territory.

New Delhi blamed that attack on what it called Islamist "terrorists" backed by Pakistan, a charge denied by Islamabad.

The crisis led to the worst military conflict between the nuclear-armed rivals in decades before they agreed to a ceasefire after four days.