A leader who attended the dinner said there were emotional moments as the prime minister went around and talked to party members with whom he worked closely over the last 10 years in running two successive United Progressive Alliance governments.
Congress vice president Rahul Gandhi was not present at the dinner held at Sonia Gandhi's 10 Janpath residence.
Official sources said Rahul Gandhi was not in Delhi and had met the prime minister last week to convey that he will not be able to attend the dinner. They said Rahul Gandhi had "already thanked" the prime minister.
The dinner was attended by union ministers, including P. Chidambaram, Kapil Sibal, Sushilkumar Shinde, A.K. Antony, Krishna Tirath, and Congress Working Committee members, including Digvijaya Singh and B.K. Hariprasad.
A leader said there were no formal speeches but Sonia Gandhi made brief remarks.
Sonia Gandhi also presented a shawl and a memento to the prime minister.
Another leader said the atmosphere inside was "happy" and the impending results of the Lok Sabha election were not discussed.
"Invitees were going to the place where prime minister, his wife and Sonia Gandhi were standing and got themselved clicked," he said.
The dinner was "lavish" and there was a variety of food items, he said.
The Oxford and Cambridge-educated Manmohan Singh has been prime minister since May 2004 when Sonia Gandhi hand-picked him for the job.
The two top Congress leaders have worked in a unique relationship with Sonia Gandhi heading the party and Manmohan Singh heading the government.
Sonia Gandhi is credited with driving the social agenda of the government.
The dinner was held two days before votes polled in the Lok Sabha election are counted May 16. The prime minister will submit his resignation to President Pranab Mukherjee May 17.
Manmohan Singh, who became India's 14th prime minister and the first technocrat to rule the country, will continue to be a member of the Rajya Sabha after he demits office.
The 81-year old prime minister had Jan 3 addressed his final press conference and announced his retirement after a decade at the helm of two successive UPA governments.