New Delhi, Aug 14 - India braced itself to celebrate its 63nd Independence Day Saturday under the shadow of heightened security threats, fears of swine flu spreading and the country heading into its worst drought in the last two decades.

Prime Minister Manmohan Singh will unfurl the national flag from the ramparts of the Red Fort and address the nation for the sixth consecutive year and is expected to focus on these pressing concerns.

While the biggest celebrations will be in Delhi, other state capitals and district headquarters will host smaller flag-hoisting ceremonies.

On Saturday, a three-tier security ring will be thrown around the Mughal-built Red Fort.

The capital will be declared a no-fly zone for the morning and air defence guns will be placed at strategic locations to ward off possible aerial attacks by unmanned or

microlight aircraft.

'India has become accustomed to heightened security provisions at this time of year but after the Mumbai terror attacks last November, the security establishment is taking no chances,' said a senior home ministry official.

As the country's top leadership is expected to be in full attendance at the Red Fort, the historic monument will be turned into a citadel. For starters, the entire Red Fort complex has already been sanitised.

This year, the threats are from multiple sources. Indian intelligence reports indicate that Islamist groups based in Pakistan and Bangladesh are planning strikes on Indian soil.