The new alert that warned of possible airborne attacks focused on three major airports - New Delhi, Bangalore and Chennai - but security was stepped up across the country. No details about the threat were released.
"This is a warning which we have received. We are prepared as usual," India's air force chief, Fali Homi Major, told reporters Thursday.
The British Broadcasting Corp. cited unconfirmed reports from airport officials as saying late Thursday that up to six gunmen had been shot and killed at New Delhi's international airport. But Indian officials told the AP there was a minor incident and no deaths.
Heavily armed guards from India's Rapid Deployment Force manned roadblocks outside airports, while others patrolled inside airport buildings among passengers.
Several extra layers of security were set up and some passengers had bags scanned with devices to check for explosives before entering terminals.
"Passengers have been asked to pass through six-stage security checks," said Brij Lal, a senior police official organizing security at the airport in the northern city of Lucknow.
Nirmala Sharma, a passenger who flew from New Delhi to Lucknow, said her bags were checked half a dozen times and she went through a metal detector three times. "Sometimes it seemed tedious, but it seems to be the need of the hour," she said.
Meanwhile, officials continued to probe the attacks.
Evidence collected in probes so far has pointed to two members of outlawed Pakistani group Lashkar-e-Taiba as masterminds in the attacks, according to two Indian government officials familiar with the matter.
The men, Zaki-ur-Rehman Lakhvi and Yusuf Muzammil, are believed to be in Pakistan, the officials said. Lakhvi was identified as the group's operations chief and Muzammil as its operations chief in Kashmir and other parts of India.
The lone surviving gunman in the assault, Ajmal Amir Kasab, 21, told police Lakhvi recruited him for the operation, and the assailants called Muzammil on a satellite phone after hijacking an Indian vessel en route to Mumbai. During the attacks, the gunmen used mobile phones taken from hotel guests to place calls to the Pakistani city of Lahore.
The Indian officials spoke on condition of anonymity because they weren't authorized to talk publicly discuss the details. Police officers said they were trying to get as much detail as possible from Kasab.
"A terrorist of this sort is never cooperative. We have to extract information," said Deven Bharti, the head of the Mumbai crime branch.
Indian police are well known for using interrogation methods that would be regarded as torture in the West, including questioning suspects drugged with "truth serum."
Bharti provided no details on interrogation techniques, but said "truth serum" would probably be used next week. He did not specify what drug would be used.
Kasab has so far told police that he and the other nine attackers trained for months in camps in Pakistan operated by Lashkar.
The revelations added to the growing evidence linking the attacks to Pakistani-based militants, and came as Secretary of State Condoleezza Rice met with leaders in Islamabad after visiting India's capital - part of a U.S. effort to pressure Pakistan to share more intelligence and pursue terrorist cells believed to be rooted in the country.
"I have found a Pakistani government that is focused on the threat and understands its responsibilities to respond to terrorism and extremism," she said after meeting Zardari.
In the meeting, Zardari "reiterated that the government will not only assist in (the) investigation but also take strong action against any Pakistani elements found involved in the attack," his office said in a statement.
He said Pakistan was "determined to ensure that its territory is not used for any act of terrorism," the statement said.
U.S. Joint Chiefs of Staff Chairman Mike Mullen, who on Wednesday was pushing the same message as Rice in Pakistan, was to meet with officials in India during his trip.
Last week's attacks were carried out by 10 suspected Muslim militants against upscale hotels, a restaurant and other sites across Mumbai.
Government security forces have come under intense criticism that they missed warnings and bungled their response to the Nov. 26-29 attacks.
On Thursday, police said an unexploded hand grenade was found outside a hospital that was the scene of an attack during last week's siege on the city. The grenade may have been left by the gunmen, but an investigation has not yet been completed, said Senior Police Inspector Shashi Pal.
The discovery came after police detected two bombs at Mumbai's main train station Wednesday, nearly a week after they were left there by the attackers.
It was not clear why the bags at the station were not examined earlier. The station, which serves hundreds of thousands of commuters, was declared safe and reopened hours after the attack. Fallout from the attacks widened Thursday as the chief minister of Maharashtra state, where Mumbai is located, stepped down. The country's top law enforcement official resigned last week.
"I regret that we could not have saved more lives, that regret will remain with me," the minister, Vilasrao Deshmukh, told reporters.
With public anger over the attacks rising by the day, Foreign Minister Pranab Mukherjee on Wednesday adopted a more strident tone against India's longtime rival, saying there's no doubt the assailants were Pakistani and their handlers in Pakistan.
Many Indians wanted more than just harsh words.
"Pakistan has been attacking my country for a long time," protester Rajat Sehgal said at a candlelight gathering in Mumbai, one of a series of rallies across India. "If it means me going to war, I don't mind."
Associated Press writers Erika Kinetz in Mumbai, Ashok Sharma in New Delhi, Biswajeet Banerjee in Lucknow and Anne Gearan in Islamabad contributed to this report.