Legal action against person trespassing private land property

I purchased a land from a lady, who had purchased it 25 years ago from a schedule caste person. Now, when I am doing construction on such land the grandson of such other schedule caste person is saying that a part of land is mine, and my grand father has'nt sold it to that lady. The person is already died, while on paper of the old sale deed between the lady and that schedule caste person the whole land is registered and sold. The question are: 1. What immediate remedy available to me in such a case. 2. As I purchased the land on a loan, and the loan is approved for land plus construction, and if I not start the construction work the bank will charge a higher rate of interest.