Fireworks erupted over the party headquarters in New Delhi drowning out the voice of his mother Sonia Gandhi as she handed over the reins of power to her 47-year-old son, who now faces the tough task of ousting the right-wing government of premier Narendra Modi.
Rahul, wearing a long flowing white kurta, smiled and waved from the dais adorned with posters of his late grandmother and father, former prime ministers Indira and Rajiv Gandhi.
"I accept this position with the deepest humility, with the knowledge that I will always be walking in the shadow of giants," Gandhi said in his acceptance speech which quickly became an attack on Modi and the ruling Bharatiya Janata Party (BJP).
"The Congress party took us into the 21st century but the prime minister today is taking us backwards to the medieval times where people are butchered for who they are, beaten for what they believe and killed for what they eat," he said.
The Modi government has been under fire in the past several months for an increase in inter-faith violence, including attacks by vigilantes largely targeting Muslims for allegedly consuming beef or killing cows which are sacred to Hindus.
Gandhi was elected unopposed to take over from his mother Sonia who has been at the helm since 1998 following the assassination of her husband Rajiv. She helped the Congress win the general elections in 2004 and 2009.
But the party was swept out of office by the BJP in 2014 and is now in a fight to win back support before a general election which must be held by 2019.
Gandhi became Congress vice-president in 2013 and led the campaign for the 2014 election, in which the party recorded its worst-ever showing as it lost power to Modi's BJP.
Since then, the party has lost polls in many key states to the BJP, exposing Gandhi to further criticism.
"Personal attacks on my son have made him fearless. I am sure he will lead the party with exemplary courage," Sonia said to loud applause from party supporters who arrived in busloads for the ceremony.
Slogans of "long live Rahul" were shouted as hundreds of supporters danced and burst firecrackers.
Gandhi will confront his first test as Congress leader this week when election results emerge from Gujarat, Modi's home state, which the BJP has ruled for over two decades.
He has led the Congress campaign for the election, winning the backing of some influential caste groups who say they have been left behind by an economic boom in the prosperous western state.
However, surveys Thursday showed the BJP would sweep Gujarat as well as elections in the northern state of Himachal Pradesh.
Results of the elections in both states will be out Monday.
Gandhi entered public life 13 years ago when he stood and won in his family seat of Amethi in north India.
The fifth-generation scion looked out of place and came across as a reluctant politician until this year when he appeared more confident during his public appearances.
Gurpreet Mahajan, a political studies professor at Delhi's Jawaharlal Nehru University, said Gandhi faces a tough task.
"It will be important how he manages to bring changes in the party functioning, like bringing young leaders to the fore and overcoming opposition criticism about his leadership," she told AFP.
"If he is able to bring changes at the grassroots level that can give a new direction to the party that has been struggling in face of a resurgent BJP."