The initiative -- in the backdrop of opposition parties joining hands to attack the government in parliament -- is a step towards strengthening the opposition and laying the foundation for a united front in the 2019 Lok Sabha elections, a party insider said.
Sonia Gandhi's invitation comes at a time when talk of a non-BJP, non-Congress front is doing the rounds following a proposal by TRS chief and Telangana Chief Minister K Chandrashekhar Rao for a national consultation process on the matter.
Sources close to the Congress confirmed that Sonia Gandhi's dinner would signify the coming together of all opposition parties who would take on the BJP inside and outside parliament.
"It would not only be a dinner, but also showcase the strength of opposition parties who would like to come together to form a front against the misrule of BJP," a senior leader said.
Several leaders have confirmed their attendance, a Congress insider said, declining to go into specifics.
However, he said Sonia Gandhi is keen to have all top opposition party leaders, including Trinamool Congress chief Mamata Banerjee, who is yet to confirm.
NDA ally TDP could also attend the Congress leader's dinner, said a senior opposition leader on condition of anonymity.
Though the TDP is an ally of the NDA, the southern regional party has joined hands with other opposition parties to protest in parliament against the centre over the issue of triple talaq. The party has also aired its differences with the government over a special package for Andhra Pradesh.
The second half of the budget session has been witnessing protests by opposition parties, which have also held demonstrations outside the House and inside.
There is "good coordination" between TRS, TDP, SP, BSP, TMC, Left, DMK and all opposition parties in Parliament on various issues, Trinamool leader Derek O'Brien said today.