Images of Mahatma Gandhi, Sonia Gandhi and Manmohan Singh beamed on a banner behind him as he accepted the takeover certificate after the traditional "Vande Mataram". Sonia Gandhi and the former prime minister were also on the stage with him.
"Congress took India to 21st century, but the Prime Minister (Narendra Modi) is taking us back to medieval times," Rahul Gandhi said, attacking PM Modi and the government in his first speech as Congress president.
"I promise to make the Congress a grand old - and young party," he told workers, switching between English and Hindi, text and extempore.
Sonia Gandhi, in an emotional farewell speech, said: "Rahul is my son, and it would not be right of me to praise him. But I can say this, that in these testing times, he has stood up to dangerous men in power. He has faced challenges with courage and will steer the party in the correct direction."
Rahul Gandhi is the fifth member of the Nehru-Gandhi dynasty to lead the Congress; no one challenged his elevation in the organization election that critics describe as a sham.
Sonia Gandhi, who took over in 1998 and is the party's longest-serving president, had told NDTV on Friday: "My role now is to retire." She also said Rahul had been playing an active role in all decisions in the past three years.
Congress leaders quickly clarified that Sonia Gandhi was retiring only as Congress president, not from politics.
Rahul Gandhi is widely expected to begin his presidency with two new defeats for the Congress, which has been struggling to win elections since it lost power at the centre in 2014.
What the Congress calls a "new era" comes two days before election results for two states that exit polls say the party won't win - Gujarat, where the BJP has ruled for 22 years, and Himachal Pradesh, which the Congress had barely won in the previous election.
