India celebrating Independence Day today

Security forces sealed off New Delhi and police scoured for militant suspects in India's financial hub Mumbai as the country feted its 61st anniversary of independence on Friday.

The national capital was turned into a fortress as security forces set up sandbag defences at key intersections and a two-hour no-fly zone was set to be enforced for the main celebrations.

Police barricaded New Delhi's 18 entry points from adjoining states and said the city borders would remain sealed during the celebrations, held every year to mark the end of British colonial rule on August 15, 1947.

Officials said the tight security was prompted by "inputs" of possible terror attacks during celebrations, which are to be kicked off by Prime Minister Manmohan Singh with a traditional address to the nation.

"We have intelligence inputs about extremist groups planning to target prominent leaders and symbols of national importance," Joint Police Commissioner S.N. Srivastava told a news conference late Thursday.

"We have taken adequate precautions," he said as elite "Black Cat" commandos were called out to join with some 17,000 paramilitary soldiers deployed to back up New Delhi's 70,000-strong police department.

The security was at its peak in Indian occupied Kashmir, which remained engulfed in deadly protests over a land row that has inflamed religious tensions and stoked insurgent’s campaign in the revolt-hit Himalayan territory.

In New Delhi, the police commissioner said "air defence measures have been also taken" with the skies above the 17th-century Red Fort declared a no-fly zone. He added that helicopter gunships would hover overhead while the premier gave his speech from behind a bullet-proof shield.

"Checking at all the borders has been intensified and spotters from other states have been deployed to identify suspects," he added.

Mumbai was also put on high alert and thousands of police searched the western seaside city for guerrilla suspects as a precaution against copycat serial blasts, which rocked the nearby city of Ahmedabad last month.

"All police officials have been ordered to be on high alert after the blasts in Ahmedabad and Bangalore," Mumbai joint police commissioner K.L. Prasad said.

At least 70 people were killed and 170 others injured in serial blasts in the two bustling cities late last month.

The national railway also stepped up security.

"All trains across the country are being guarded," said a spokesman for the rail network, which transports 13 million passengers daily and is often a militant target.

Fifteen of India's 29 states battling outlawed Maoists for half a decade have also cranked up security, officials said.