India's capital and its other major cities have been placed on an unprecedented security alert amid threats of terror attacks ahead of the country's Independence Day celebrations, officials said Monday.

The security alert came amid intelligence reports that Islamic militant groups were plotting major attacks around the Tuesday's Independence Day celebrations. Last week, the U.S. Embassy in New Delhi warned that groups linked to Al Qaeda could target hotels, airports or historic monuments in New Delhi, Bombay and other major cities.

Tens of thousands of policemen and paramilitary soldiers were deployed across the Indian capital, a city of 14 million people, where terror attacks were feared the most. Sharpshooters stood atop some government offices, while security forces placed barricades in many major streets of the city.

New Delhi will be declared a no-fly zone for several hours when Prime Minister Manmohan Singh hoists the national flag from the Red Fort — a 17th century sandstone structure built by Mogul emperors, said a Home Ministry official.

Roads leading to the Red Fort will be shut to normal traffic then, said the official, who didn't want to be named because of the sensitivity of the issue. Airports across the country also were on high alert, he said.

In Bombay, where security has already been tightened after deadly July 11 train bombings, authorities deployed additional forces in what they described as "sensitive areas" — a euphemism for Muslim-dominated neighborhoods.

Bombay's Police Commissioner A.N. Roy said police held meetings with owners of shopping malls and cinema halls to discuss extra security measures, as intelligence reports have said those could be targeted by terrorists. Temples and mosques have been asked to take more precautions, Roy said.

Elite commandos were guarding nuclear facilities such as the Bhaba Atomic Research Center in Bombay and the Kalpakkam nuclear reactor in Tamil Nadu, the Press Trust of India said.

In strife-torn Jammu-Kashmir state, officials said they had intercepted communications among militant groups planning major attacks.

Several Islamic rebel groups, which have been fighting for Kashmir's independence from India, have called for a boycott of the Independence Day.

"Militants would like to show that they can disrupt the (Independence Day) celebrations, but we have taken adequate measures ... to thwart any attack," Deputy Inspector General of Police Farooq Ahmed.

Ahmed said authorities had held security drills in and around Baxi Stadium, where the state's elected head was to hoist the national flag during Tuesday's festivities.

There are threats from other groups as well — separatists in the country's northeast and Maoist groups in some 13 states in eastern and southern India.

On Sunday, the government announced a 10-day unilateral cease-fire with a leading separatist rebel group in the northeastern state of Assam, in a bid to placate militant attacks. Chief Minister Tarun Gogoi, the state's elected head, said security forces would still a keep a strict vigil.