Illegal and Arbitrary claim of ownership by ex-owner after 30+ years of owning and possesion of land

My Dad and his friend co-owned an empty plot for last 25 years in Delhi, It was used for gardening and had an electricity meter since the begining. 2 years back we decided to split the land into two parcels, and sold the property to each others son to transfer ownership of land. Now, and ex owner of the land who owned that entire block 30-40 years ago with old paper is claiming the ownership (After 30+ years of peaceful/ conflict free ownership) and disrupting construction. This is a usual trend in the area by people who used to have farmlands in the area 40-50 years ago. The plot lies in an unauthorised colony area but it is the only vacant non- residential plot in the area. The local police are also taking advantage of the situation by getting involved. I am aware I have plenty of proof to show my/our right on the land, but I want to avoid long lasting court case and all the hassle related to it, as it is totally unnecessary. Could you please provide some expert opinion on how to deal with this.