Ministers had already claimed the blast, which killed 11 people and injured 62, was a "terrorist attack".
The bomb hit a packed reception area where more than 100 people were queueing for their entry passes to the court in New Delhi on Wednesday. Many of the victims were waiting for their cases to be taken.
Police said the device had apparently been placed in a briefcase near the reception to the court complex.
There was no immediate claim of responsibility.
"This is a cowardly act of a terrorist nature," Singh, who is currently on a visit to Bangladesh, told television reporters in Dhaka. "This is a long war in which all political parties, all the people of India, have to stand united so that is crushed."
Home Minister P Chidamabaram, in a statement to parliament, called on people to remain "resolute and united" in the face of efforts to destabilise the country.
One police source said initial investigations suggested the bomb had contained potassium nitrate.
The High Court was targeted in May this year when a low-intensity device was set off in the parking lot. There were no casualties and only minimal damage.