The president from 2002 until 2007, he was a scientist and science administrator for four decades before that, mainly at the state-run Defense Research and Development Organization and the Indian Space Research Organization. He played a key organizational and technical role in India's nuclear program.

He also served as a scientific adviser to the prime minister.

Kalam fell sick Monday while addressing the students of the Indian Institute of Management in the Meghalaya state capital, Shillong, said P.B. Warjri, chief secretary of the state. He was rushed to the hospital, but was declared dead after doctors could not revive him, Warjri said.

The federal government would declare national mourning for seven days for him, said federal Home Secretary L.C. Goyal.

Kalam's body will be flown to New Delhi on Tuesday for burial.

India's President Pranab Mukherjee said "Kalam would be long remembered for his passion for science and innovation and his contribution as an eminent scientist, administrator, educationist and writer."

Prime Minister Narendra Modi said "his death is a great loss to the scientific community. He took India to great heights. He showed the way."

"He was an inspiration to an entire generation," Home Minister Rajnath Singh tweeted.

Sonia Gandhi, the president of opposition Congress party, described Kalam as "one of the greatest scientific minds, a scholar statesman and a true patriot who inspired millions of young and old alike by his works and deeds."

After his presidential term ended, Kalam traveled to remote schools and colleges, speaking to students and inspiring them to follow their dreams.

Even after he left office, he received hundreds of emails each day from high-school students and other young people seeking his advice. He would reply to most of the children who wrote to him.

Kalam received several prestigious awards, including Bharat Ratna, India's top civil honor conferred by the government.

He played a crucial role when India tested its nuclear weapons in 1998.

He authored several books, notably 'Wings of Fire', 'India 2020' and 'Ignited Minds.'

Kalam had a humble beginning. His father owned boats which he rented out to local fishermen. Kalam used to describe himself as "Made in India," as he never trained abroad.