They paid floral tributes at the slain leader's memorial at Shakti Sthal here.
"Remembering Dadi (grandma) today with a deep sense of happiness. She taught me so much and gave me unending love. She gave so much of herself to her people. I am very proud of her," Rahul Gandhi tweeted.
"Today we honour... the first female Prime Minister of India and one of the strongest leaders our country has seen," the Congress party said.
"Through her premiership, our nation saw great victory, incredible development and most importantly upliftment of all sections of society," the party tweeted.
(With inputs from agencies.)