Whoever assaults or threatens to assault, or obstructs or attempts to obstruct, any public servant in the discharge of his duty as such public servant, in endeavouring to disperse an unlawful assembly, or to suppress a riot or affray, or uses, or threatens, or attempts to use criminal force to such public servant, shall be punished with imprisonment of either description for a term which may extend to three years, or with line, or with both.