What is the difference between pre-grant and post-grant opposition in India?

India offers both, and they are separate proceedings before the Patent Office.

Pre-grant opposition (Section 25(1)) may be filed by any person, at any time after the application is published and before a patent is granted. It is a representation to the Controller.

Post-grant opposition (Section 25(2)) may be filed only by a person interested, as defined in Section 2(1)(t), and only within twelve months from the date of publication of the grant. It is an inter partes proceeding, with a statement and evidence, an Opposition Board constituted under Section 25(3), and a hearing.

The grounds available are the same eleven grounds in both provisions — including wrongful obtaining, prior publication, prior claiming, prior public knowledge or use, obviousness, subject matter that is not an invention or is not patentable under the Act, insufficiency of description, incorrect or false information under Section 8, and non-disclosure or wrongful mention of the source or geographical origin of biological material.