Since January 2005, India has had a post-grant opposition system to challenge a granted patent in India. Under this, you can oppose a granted patent within 12 months of the announcement of the decision to grant.
Interested parties, or the government, can also file a petition for revocation of a patent at any time during the life of the patent. Such petitions must be lodged with the Intellectual Property Appellate Board (IPAB) or at High Court level.
Along with the opposition proceeding, you should conduct a patent invalidity search as well to reinforce your claim. Check out the MANUAL OF PATENT OFFICE PRACTICE AND PROCEDURE (Chapter 10 and Chapter 11) to see the details of the procedure. This will help you to prepare to challenge a granted patent in India.
Also, do make sure that you check out this free e-book: A Quick Guide to Patents. Basically, it deals with basics of patent procedures.
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