In his first official address, he criticized BJP and the Modi-government for "spreading fire and violence" in India.
He said the Opposition will now defend every Indian's voice and be the "bastion of people".
Manmohan Singh, Sonia Gandhi reiterate "disturbing trends" in India
At Rahul's oath-taking ceremony, former PM Manmohan Singh and Sonia Gandhi expressed concerns over "disturbing trends" in India, amid "politics of fear", taking a dig at BJP.
In her final speech as Congress's Chief, she urged the party to work to "safeguard values on which the country was founded."
She also said she was retiring as the Congress President but not from politics.
Rahul Gandhi will overcome challenges
On Rahul Gandhi taking over as the President of Congress, senior party leader Ghulam Nabi Azad said: "I am confident he will prove to be a good party president. He will face and help overcome the challenges the country is facing today."
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I'm an idealist, like many Indians: Rahul Gandhi
Rahul accused BJP of spreading violence, adding the Congress "will fight for those...who can't fight alone."
He said BJP is taking India back to "medieval times." He said, "Today, politics is being used to crush people," but Congress will fight the politics of fear and anger.
Rahul said Congress would become a "grand old and young party," transforming India into a "country of brotherhood".
BJP's power is manipulative, distortive: Rahul Gandhi
Attacking the Modi-government, Rahul said: "Their (BJP) power is manipulative and distortive. We are now made to believe that only one man is strong. They attack the Congress and try to weaken us. Stand up to them...their anger and hatred makes all of us stronger."
This is a new era, a new leadership: Sonia Gandhi
Addressing the gathering, Sonia Gandhi said Congress members "aren't the ones to get scared, or bend"; their only goal is to protect India.
Attacking BJP, she said the "country's foundations and shared culture" are under attack, and there is an "atmosphere of fear".
She said Rahul had adopted ahimsa (non-violence) from childhood. She said he would lead Congress with "a pure heart and dedication".
Rahul would transform politics of fear to hope: Manmohan Singh
Speaking at the ceremony, former PM Manmohan Singh said Rahul Gandhi "brings with him a new sense of the leadership road with courage and humility."
He added Rahul is taking over Congress at a time when there are dangers that "politics of fear" would replace "politics of hope".
He also said Sonia Gandhi's contributions to Congress would be remembered in the party's history.