It was not possible at this stage to identify the terrorists who exploded a bomb outside Delhi high court on Wednesday killing nine people, home minister P. Chidambaram said.
Chidambaram told the Lok Sabha that Delhi was a target for terrorists as it was the national capital.
Accordingly, Delhi had been put on high security alert, he said in a brief statement.
The minister said the blast left nine dead and some seriously injured.
"We are clear that there is no cause that will justify the terrorist act," he said.
Home Secretary R.K. Singh said investigating teams were already at the blast site. "We have the National Investigation Agency team right there. We have people from the forensic laboratories there, we have NSG (National Security Guard) team there to collect evidence."
The investigation has been handed over to the NIA.
Special Commissioner of Police (Law and Order) Dharmendra Kumar told reporters that police had cordoned off the area and were requesting people not to converge at the site.
Wednesday's blast comes after the July 13 triple bombings in Mumbai that killed over 25 people. Before that, in February 2010, an explosion inside a popular eatery in Pune claimed 17 lives, four of them foreigners.
The terror attack Wednesday is also the second time in four months that the Delhi High Court has been targeted. On May 25, a low intensity explosion took place without causing any damage. The explosives were hidden in a plastic bag near a car in a service lane outside the court.
This is also the 19th blast in the capital, in 15 years.