Sonia was also joined by son and Congress vice president Rahul Gandhi, daughter Priyanka and son-in-law Robert Vadra.
'Shanti Mantra' was recited on the occasion as youth holding the Tricolour circumambulated the memorial.
A number of young children also offered floral tributes at the memorial.
Delhi chief minister Sheila Dikshit, external affairs minister Salman Khurshid, defence minister AK Antony, parliamentary affairs and urban development minister Kamal Nath were among those who paid tributes to the leader.
Gandhi was assassinated on May 21, 1991 at Sriperumbudur in Tamil Nadu during a poll campaign.