New Delhi - Indian police on Sunday were hunting for bombers involved in multiple blasts in New Delhi that killed at least 21 people, the worst terrorist strike in the country since 56 people were killed in the western city of Ahmedabad in July.

Five explosions ripped through busy shopping and commercial areas on Saturday evening, killing 20 people and injuring 98.

A 15-year-old girl succumbed to injuries on Sunday, raising the death toll to 21.

Raids were being conducted across the city as special units from the Delhi police detained 12 suspects for questioning in connection with the attacks.

Senior police officials said initial investigations showed that the explosives and timers used in the bombings were similar to the July blasts.

Delhi police spokesman Rajan Bhagat confirmed that several people were detained.

"Several suspects are being questioned, but no arrests have been made so far," Bhagat said, adding that police had obtained "vital leads."

Bhagat said the police were preparing sketches of men who planted the bombs based on descriptions given by witnesses.

"Several police teams have fanned out to various places in Delhi and its neighbouring states for the probe," Bhagat said.

Investigators also traced an e-mail sent by a Muslim militant group called the Indian Mujahideen which claimed responsibility for the attacks to the western Mumbai city.

Security agencies believe that the Indian Mujahideen is a front of the banned terrorist group Students Islamic Movement of India (SIMI), and Pakistan-based Lashkar-e-Toiba, which have jointly carried out attacks in the past.

The Indian Mujahideen had claimed responsibility for the serial bombings in Ahmedabad and others in the northern Jaipur city in May which claimed 63 lives.

The group said it had declared an "open war" against India in retaliation for atrocities committed on Muslims and the country's support for US policies.

Indian news channels reported that police suspect that SIMI activist Abdul Subhan Qureishi, a software engineer from Mumbai, was behind the attacks in Delhi and Ahmedabad.

Intelligence sources told the NDTV news channel that Qureishi had come to Delhi in July to recruit men and handed over explosive material.

Two of the blasts hit the Connaught Place commercial area in the heart of Delhi, while two more exploded at the upmarket shopping district of Greater Kailash.

The bombs were planted in dustbins and bicycles. Four unexploded bombs were also found and defused on Saturday night.

Relatives thronged city hospitals where victims had been admitted.

"We are frightened but angry at the same time," Ashu, a college student whose relative was admitted at the Ram Manohar Lohia (RML) hospital, told reporters.

"These enemies of humanity, whether they be Muslims or Hindus, should be eliminated so they never get a chance to carry out such attacks again," he said.

Later on Sunday, Prime Minister Manmohan Singh visited the RML hospital to enquire about the condition of the blast victims and spoke with doctors attending to the injured.

Indian cities remained on high alert as additional police were deployed around vital installations, religious places and market areas in major cities.

There was an uneasy calm in Delhi on Sunday as police increased patrols and erected barricades at various parts in the city.

The usual Sunday crowds at popular shopping areas and cinema halls were missing and the markets which were targeted remained shut.

A tight vigil was being maintained at the airports, metro stations, malls, hospitals and other crowded areas.

The main opposition Bharatiya Janata Party has criticized the Manmohan Singh government for its failure to tackle terrorism and called for stronger anti-terrorist laws.

India is among the countries worst affected by terrorism. More than 650 people have been killed in terrorist attacks in Indian cities over the past eight years.

The deadliest terrorist attack in recent years was in July 2006, when multiple bombings hit Mumbai's train network, killing 180 people.