Describing Indira as "one of the greatest" prime ministers, Sonia Gandhi said there was only one religion for the late Congress leader -- that all Indians were equal children of the motherland.
Former president Pranab Mukherjee, ex-prime minister Manmohan Singh, Congress vice president Rahul Gandhi and his cousin -- BJP MP Varun Gandhi also remembered Indira Gandhi, often described as the 'Iron Lady of India'.
"Nation remembers former Prime Minister Smt Indira Gandhi on her birth centenary #PresidentKovind," Kovind tweeted.
Taking to the micro-blogging site, Modi said, "Tributes to former PM Mrs Indira Gandhi on her birth anniversary."
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The Congress president said Indira Gandhi fought for secularism and against all those forces seeking to divide the people on the lines of religion and caste.
Indira Gandhi was assassinated at the Safdarjung Road bungalow, her official residence, by her bodyguards on October 31, 1984.
The Congress president said Indira Gandhi took pride in India's rich diversity and its democratic and secular values.
The former prime minister also fought for India's dignity and independence as a sovereign nation, against the dominance of superpowers, she said.
Terming Indira as a leader of "courage and conviction", Mukherjee listed several of her decisions and stands she took on various issues, including Operation Blue Star and nationalisation of banks as a prime minister.
He also sought to underscore that the late leader was very concerned about promises made to the people and their actual delivery.
"She provided this by emphasising that the Congress must stick to its ideology," he said during his address.
To drive his point that her memory cannot be erased, Mukherjee invoked late British politician Michael Foot who in 1978 had said, "The Indira Gandhi story has not come to an end after the defeat in general elections of 1977 as many people believe she is doomed forever.
Mukherjee said, "In her death, she left a message to the people of this country and to the whole world: a commitment to the people can never be killed by any power, however strong it may be."
Mukherjee, Singh and Rahul paid floral tributes to her early in the morning at her memorial, 'Shakti Sthal', as well.
Rahul described his grandmother as his mentor and guide. "I remember you Dadi with so much love and happiness. You are my mentor and guide. You give me strength. #Indira100," he tweeted.
He also tweeted a picture in which the former prime minister is seen holding him, then a child, in her lap.
Congress' communications in-charge Randeep Surjewala said on Twitter that she worked tirelessly "to shape a new future" for the country.
Indira Gandhi was born on this day in 1917 in Allahabad in Uttar Pradesh.