Pakistan's minister of state for information, Tariq Azim, immediately denied the allegation, calling it "irresponsible" and demanding that India provide evidence of the link.
India called a halt to the often-stumbling, two-year-old peace talks with Pakistan in the wake of the bombings, which ripped through a series of suburban commuter trains during evening rush hour on July 11, killing at least 207 people and wounding 700.
Negotiations resumed earlier this month when Indian Prime Minister Manmohan Singh met Pakistani President Gen. Pervez Musharraf on the sidelines of a conference in Cuba. The two agreed to set up a joint mechanism to combat terrorism.
Mumbai Police Commissioner A.N. Roy, the lead investigator in the bombings, said Pakistan's Directorate of Inter Services Intelligence, or ISI, began planning the attacks in March and later provided funding and training for the bombers in the Pakistani town of Bahawalpur, a center of militant Islamic activity.
"The conspiracy was hatched in Pakistan," he said at a news conference.
Roy said the attacks were carried out by the Pakistan-based Islamic militant group Lashkar-e-Tayyaba, with help from the Students Islamic Movement of India, a banned Muslim organization.
He said 15 people have been arrested so far, including 11 Pakistanis. Three Indians are still on the run, he added, and another Pakistani bomber was killed in the blasts.
Roy offered no evidence to support the link, but said it was revealed during the questioning of suspects who had been drugged with a "truth serum" to force them to divulge information. He didn't describe what the drug was or how it worked.
Top Indian government officials did not comment on Roy's accusation.
Pakistan has in the past denied involvement in the attacks and it was not immediately clear how the revelations would affect the fragile peace process. A 2001 attack on India's Parliament, blamed on the same agency, almost pushed India and Pakistan to war.
"Whenever some bad thing happens in India, they start blaming us for it," said Azim, the Pakistani official. "Such allegations only give benefit to the real culprits, who escape arrests."
Spokesmen for the outlawed Lashkar-e-Tayyaba group were not available for comment.
The allegation came days after the British Broadcasting Corp. reported that a document prepared for Britain's Defense Ministry accused ISI of indirectly supporting terrorist groups including al-Qaida. Musharraf strongly rejected the leaked document.
Roy described an elaborate network that helped the militants carry out the bombings.
He said they slipped into India after training in Bahawalpur, some directly from Pakistan, others through neighboring Nepal and Bangladesh. They were then met by Indians who brought them to Mumbai and housed them in rented apartments, Roy said.
The bombs were made by packing smuggled high-grade RDX explosives with ammonium nitrate into pressure cookers. Teams of two — one Indian and one Pakistani — took the bombs onto the trains where they were set off by timers, he said.
"It was a professional, precise and well-planned operation," he said.
Police cracked the case after tracing a suspicious call from Mumbai, formerly known as Bombay, to the Nepal border region, Roy said. There, they picked up one of the suspects, who led them to others.
Pakistan and India have a long history of bitter relations and their border remains heavily militarized. They have fought three wars — two over the divided Himalayan territory of Kashmir — since gaining independence from Britain in 1947.
Lashkar-e-Tayyaba is among more than a dozen Muslim groups that have been fighting since 1989 for Kashmir's independence from India or its merger with Pakistan. More than 68,000 people have been killed in the conflict.