Jha had alleged that Ajay and his family members have encroached upon the land belonging to tribals at their ancestral place of Churhat.
The suit was filed in the court of the district and session judge Sushma Khosla by Ajay through his counsel Vineet Godha.
Ajay had already filed a separate complaint demanding registration of the criminal cases against Jha and others in the court of First Class Magistrate for levelling false allegations against him on the basis of fake documents.
Jha had on April 12 lodged a complaint with the state Commission for SC and ST against Ajay Singh, saying that the latter grabbed land owned by tribals. He also alleged that he had documentary evidence to prove that Ajay's kin were in possession of the government land.
Besides Jha, the defamation suit was also filed against BJP's state general secretary Nand Kumar Singh Chouhan, MLA Kunwar Singh and the party secretary Shardendu Tiwari.
The petitioner claimed that due to their "baseless" allegation, his reputation and that of his family members has been damaged, and sought Rs 10 crore in compensation.