builder delayed possession of flat

please guide me on how to take legal action against a very big builder in country. They have delayed possession of flat for more than 1.5 years, still awaiting I booked flat on March 2012, possession was promised Oct 2014. On top of that, they have put interest penalty on me for delayed payment, which is even more ridiculous as they delayed the slab completion which prevent bank to release installment, on time. There are many more issues. Can someone please give me some advice on how to take legal action. Please? There are many people who as facing issues with this builder but i don't know why no one things of taking any action.