False ownership claim by Grandsons.

Sir, I am a senior citizen aged 70 years. I have puchased a 1/2 acre land from 'A' in the year 2005 in Andhra pradesh after veryfying the passbooks, Adangal, His registred document of the year 1990, and Encumberence certificate. Now after 11 years as the prices have abnormally increased some group of say that we have also got right on this property. Thier explanation is as follows:- The property is an inherited property and A has managed alone to transfer the property and got it registered in 1990 from his father X. who have three children A,B,C. Now all X,A,B,C died. While I purchased the land after veriying the documents and registered and transferred from A his children also have signed. But now the children of B and C says that we too have got right on this property. They do not possess any sort of documents with them. I observed some group of people who are doing malpractices including the village head are involved and insisting me to sell the property for a much lesser price.As they are local people they are harassing me to sell the same or else they will create documents in thier own ways. Now as all the revernue documents are clear in my name, is it possible for them to manipulate the documents in the revenue records. Do they really have rights to claim? How should I proceed for complaint and whom to complain and what are the supporting documents to be enclosed with the complaint? I will be very greatful to you can show the direction in this regard.