Shakti Sinha, former bureaucrat and academician who was private secretary to late prime minister Atal Bihari Vajpayee, passed away on Monday. He was 64.

According to sources, Sinha was suffering from high blood pressure and was rushed to RK Hospital in IP Extension when he complained of uneasiness.

He collapsed at the hospital, the source said.

Political leaders and other dignitaries expressed shock and grief over Sinha's sudden demise and recalled his contribution as a strategic thinker and leader in public policy.

BJP's National Vice President Baijayant Jay Panda also condoled Sinha's demise. He described Sinha as one of the finest civil servants and public intellectuals.

He penned down a heartfelt note on Twitter and paid his tributes. His post read: "Shakti Sinha was one of our finest civil servants & public intellectuals. He had served as a close aid to PM Vajpayee, wrote & spoke extensively, & was widely liked & admired. He passed away suddenly today, far too early for he had a lot more to contribute to India. Om Shanti"

"How fragile the life is! Met Shakti Sinha ji only yesterday and had a long and enriching interaction. Now he's no more. Deeply distressing!," Ajay Singh, Press Secretary to President Ram Nath Kovind, tweeted.

BJP leader Ram Madhav said Sinha was a member of the Governing Board of India Foundation and was scheduled to address a conference at Leh this afternoon.

"A humble and unassuming yet erudite and intellectual being a great loss. Deepest condolences. Aum," he tweeted.

Sinha, an IAS officer of the 1979 batch, was also former Director of Nehru Memorial Museum and Library (NMML).

"The Press Club of India is shocked at the untimely passing of Shakti Sinha, former bureaucrat and Director of Nehru Memorial Library and Museum at Teen Murti, academician and author. Mr Sinha was part of a PCI discussion just a few days ago. We mourn his untimely demise," the PCI tweeted.

Sinha worked closely with Vajpayee during 1996-1999 and had written a memoir titled 'Vajpayee: The Years That Changed India'.

Former Army chief Ved Malik said Sinha was always helpful on matters concerning the welfare of armed forces.

"Sad to learn about the passing away of Shakti Sinha. A scholar and a good man. Was PS with PM Vajpayee during Kargil war. Always helpful on matters concerning welfare of armed forces. RIP. Om shanti," he tweeted.

Arunachal Pradesh Chief Minister Pema Khandu expressed sadness at Sinha's demise.

"We lost a very eminent person. His wonderful work & gentle soul will forever remain in our heart. I pray Lord Buddha for strength to the bereaved family. May the departed soul attain Moksha," he tweeted.

The IAS association tweeted, "we condole the untimely demise of Sh Shakti Sinha, #IAS AGMU:79. He was an Administrator, Thinker and Author par excellence.

"Recently he published widely acclaimed book- Vajpayee: The Years That Changed India. Our condolences to the family members".

Meanwhile, a condolence meeting was held at Indian Embassy in Moscow.

"Condolence meeting was held in the Embassy to mourn the sudden demise of Shri #ShaktiSinha; Ambassador and other speakers recalled his recent visit to Russia to promote work of the International Buddhist Confederation," the Embassy tweeted.