Congress vice president Rahul Gandhi and Aam Aadmi Party leader Arvind Kejriwal also voted in separate polling centres.
Sonia Gandhi was accompanied by Congress leaders Ajay Maken, Arvinder Singh Lovely and Haroon Yusuf when she reached the Nirman Bhawan polling booth in the heart of the capital.
After voting, a smiling Gandhi flashed her inked finger to the media.
Kejriwal told journalists after casting his vote that he was confident that the AAP would win all seven seats in Delhi.
Earlier, Ajay Maken made a similar claim for the Congress, saying voters would reward the Congress-led UPA with another five-year term.