Under Section 25(2) of the Patents Act, 1970, after the grant of a patent, who may give notice of…

Section 25(2) lets "any person interested" give notice of opposition "at any time after the grant of patent but before the expiry of a period of one year from the date of publication of grant of a patent". Opposition by "any person" is the pre-grant route under section 25(1), and the period is one year, not six months or two years.