The 47-year-old was officially declared party president in a ceremony held at the party's 24 Akbar Road headquarters here.
He was given the certificate of Congress president by Mullappally Ramachandran, the chairman of the party's central election authority, who oversaw the organisational polls.
The ceremony was attended by party stalwarts, including former prime minister Manmohan Singh, former union home minister Sushil Kumar Shinde, former Maharashtra chief minister Ashok Chavan and other party leaders.
Sweets were distributed, slogans raised, crackers burst and party workers danced to drum beats to celebrate the change of guard. Artists from different parts of the nation were also seen performing outside the party head office. Congress workers marched from Rail Bhavan to party office. Sonia headed the party for 19 years since 1998.