How can I challenge a granted patent in Japan?

Between 1 January 2004 and March 2015, the only way to do this was via invalidation proceedings. However, opposition was re-introduced on 1 April 2015, so now two procedures are available again. Invalidation requests challenging one or more claims can now only be filed by interested parties, but any time after grant (and even after the patent has expired). The recently re-introduced opposition procedure can be initiated by anyone, but only within six months as from publication of the granted patent.