Land Grabing & illegal Construction

Dear Sir, I live in Worli Koliwada. We have purchased our house in a chawl from so called landlord, who is a local fisherman in the year 1980. Our & local houses are G+1. There is some free land in the area which we use to play, park bike and all. Now there is another fisherman who claims this free land is his property & he wish to use this land for fishery work. I have heard that 25 -30 yrs back there were few lands this fisherman use to use for drying fishes. Slowly they started building the houses in same land & sell it to tenants. Now this land is free from past 25 - 30 yrs & this fisherman is claiming that this is his property and he will use it for his work. 5 yrs back he has constructed G+1 floor building on such land around. We are in knowledge that this local land lords are holding property card for only the houses they stay. Rest all construction done is on Vacant land / Goverment land which they used to use for fishery work. They come to the land have the fight with our local tenants for that land, on asking them for property papers they start fighting, which clears that they dont posses the land. What shall we do? our whom shall we approach for help.