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.
