Trespassing / Land Encroachment / land grabbing / fake / illegal possession

My father is a veteran with 28 yrs of Armed Forces service, after his retirement he purchased a piece of NA layout land which is under gram panchayat, and since then it was unattended for 20yrs,(no construction was done) but regular clearing of undergrowth was done. A month prior, one of the member of gram panchayat was clearing that land with JCB, soon he was stopped by my father claiming the property owner, this person also claimed the property stating "property belong to him" soon he was aggressive and stated pushing the senior citizen(my father) soon he was stoped by the neighbouring ppl and had to return the same day. My father immediately informed the local police with a complaint application against the individual. Event after multiple visits no FIR registered by police. Meanwhile my father also checked with Sub registrar office for the property papers and ownership, it also reflected my father name. After enquiry we found out that this person is perpetual defaulter and indulge into many such cases of land grabbing by trespassing by producing fake documents and have the backing of administration and police. And he also managed to pay tax to gram panchayat office for the same property for last 10 yrs as he is one the member. PDO is also favoring him. Meanwhile, with the consent of the police station my father constructed a metal shed at the property but this individual not only interrupted the process but also verbally abused and threaten them. Police was agin intimated the same but was not effective. Police called both the parties and asked my father to stop the work and the other individual to produce his documents in a week. My father produced the documents to the inspector but even after one week this individual did not turn up with any documents, still no action by the police Soon after a week this individual again pasted a printout/ cutout at the shelter which we build stating "This proper belong to him with his contact number" which is removed by my father next day again he did the same and this rat chase continued. Police was adequately informed but no action. Request to pay tax for all these years is given to PDO at gram panchayat, which is still pending. Please do let me know whom to approch ,