India was on high alert Tuesday as it celebrated 59 years of independence amid threats of terrorist attacks that the prime minister said undermined a 2 1/2-year peace process with Pakistan.

Intelligence reports have indicated that Islamic militant groups might be plotting major attacks around Tuesday's celebrations. Last week, the U.S. Embassy warned that groups linked to al Qaeda could target hotels, airports or historic monuments in New Delhi, Bombay or other cities.

In his Independence Day address to the nation, Prime Minister Manmohan Singh said terrorists want to derail India's booming economy, destroy the unity of its people and spark religious strife.