The party also remembered Sardar Vallabhbhai Patel whose birth anniversary falls on the same day.
"On this day, Indira Gandhi laid down her life, fighting for India's unity and integrity. We salute her martyrdom," the Congress party tweeted.
Gandhi was India's first woman Prime Minister. She was born on November 19, 1917, and was assassinated on October 31, 1984, a few months after she ordered the Operation Blue Star in Amritsar to flush out Khalistan terrorists from the holiest of the Sikh shrines, the Harmandir Sahib, better known as the Golden Temple.
She was the second longest-serving Prime Minister after her father, Pandit Jawaharlal Nehru, the country's first Prime Minister. She held the office from January 1966 to March 1977 and from January 14, 1980, till her assassination.
Indira Gandhi was conferred the Bharat Ratna in 1971 by then President V.V. Giri.
"We pay homage to Smt. Indira Gandhi and Sardar Vallabhbhai Patel today (Monday)," the Congress wrote in another tweet.
Vallabhbhai Patel was born on October 31, 1875, in Nadiad, a small village in Gujarat. He was conferred with Bharat Ratna in 1991 for his services to the nation. He played a leading role during the freedom movement and later in successfully integrating hundreds of princely states into a united, independent state.
He was India's first Deputy Prime Minister and the Home Minister. He was popularly known as the 'Iron man' of India.