Section 25 (1) speaks of pre-grant opposition to a patent which must be filed within three months of the publication of a patent and is heard by the Controller.
Section 25 (2) speaks of post-grant opposition to a patent which must be filed within one year of the grant of a patent by a person interested (as defined by Section 2(t))
Section 25 (3) makes provision for the formation of an Opposition Board to hear post-grant opposition.
Under Section 25 (4), the Opposition Board may maintain, amend or revoke a patent after it has heard the opposition to the grant of the patent.
Rule 55 of the rules made under the Act also applies to the opposition to a patent.

Grounds of Opposition
1. that the applicant or the person under or through whom he claims has wrongfully obtained the patent
2. that the invention so far as has been claimed in any claim in the complete specification has been published before the priority date of the claim (i) in any specification filed in pursuance of an application for a patent made in India on or after January 1, 1912 or (ii) in India or elsewhere in any other document (subject to statutory exceptions)
3. that the invention is claimed in an application filed in India with a priority date earlier than that of the applicant
4. that it was publicly known or used in India before the priority date of the applicant’s claim (or in the case of a process, that a product made by it was imported into India for purposes other than reasonable trial or experiment before the priority date of the applicant’s claim)
5. that the invention does not involve an inventive step i.e. that it was published earlier or that it was used in India before the priority date of the applicant’s claim
6. that it is not an invention or that it is not patentable under Sections 3 or 4
7. that the complete specification does not clearly or sufficiently describe the invention or the method to produce it
8. that the applicant has failed to disclose information required to be disclosed under Section 8 to the Controller or knowingly furnished material which is false in any material particular
9. that the source or geographical origin of any material is wrongly mentioned or is not mentioned in the complete specification
10. that the invention was anticipated considering knowledge in any local or indigenous community in India or elsewhere
11. that, in the case of a Convention Application, it was not made within the prescribed 12 months from the date of the first application.