Received faulty product from online store, refusing refund, what to do now?

I recently bought a smartphone online from a reputed e-commerce website. It cost me ₹25,000. The phone started malfunctioning within a week. I've contacted the customer support several times, but they keep delaying. Now they are refusing to grant a refund or replacement. I'm feeling frustrated and cheated. I live in Chennai, and I want to know whether I can file a complaint against them and how to proceed with this. What are my rights as a consumer?