Land Dispute

My father, who is now 65, had purchased a plot 60*30 in the year 2004 from a local seller (Second Party) by paying the value of the land along with the stamp duty and registration charges, in Gulbarga District, Karnataka. That seller (Second Party) had bought the same plot from another Seller (first Party) in the year 1995 with all necessary sales deed and registration on Rs. 5000 stamp paper. The legal heirs of the first party at that time (in the year 1995) were minors, so their mother processed the sale deeds on the behalf of her sons' with the second party. Now, those sons' (the first party) who does not possess any legal documents at all and with the help of local goons and some police officials claiming that the property belongs to them and refraining us to do any kind of construction on the plot. So, in these, what should be the course of action? We are also planning to file a case of harassment on senior citizen (my Father, aged 65) and cheating, against them (first party). Are such laws available in IPC? Can we file compensations case against them? Because it is mentioned in the sales deed that if in future any third person claims the land for any reasons then the seller with compensate (the current value of the land) to the buyer. All the documents are in our possessions and they (first party) has some forged property tax bill with them on which they are claiming the property. Thank You.