A letter from a previously unknown group outlining the threatened attacks in Bangalore was faxed to several newspapers late on Thursday, B.S. Sial, director-general of Karnataka state police, said on Friday.
Bangalore, the capital of Karnataka state, is India's technology and software hub - home to about 1500 software companies.
"It will be the most coordinated attack the country has ever seen," independent television channel CNN-IBN quoted the letter, written in English, as saying.
Six attackers will trigger explosions including, "two human bombs to target the state chief minister."
Moin-ud-Din of the Al-Jehadi group said, according to the report.
"Newspapers have received the letter late last night, and we are trying to find out if it is a hoax. But we will not take any chances," said Sial, without giving further details.
The letters were sent a day after gunmen opened fire on Wednesday outside a prestigious science institute in Bangalore, killing a retired professor and wounding four others.
"Newspapers have received the letter late last night, and we are trying to find out if it is a hoax. But we will not take any chances"
B.S. Sial, director-general of police, Karnataka state
Police set up barricades, patrolled streets and continued to search cars at checkpoints on Friday across the city, hunting for the attackers, Sial said.
Police suspect Kashmiri rebels were behind Wednesday's attack, but no group has claimed responsibility.
More than a dozen rebel groups have fought in Indian Kashmir since 1989 to wrest a separate homeland or merge with neighbouring Pakistan. The insurgency has left 66,000 people dead.