Builder demanding extra ₹3 lakh before giving flat possession, is this legal?

I booked a flat in Mumbai with a reputed builder and paid all installments as per the agreement. Now when the possession date is here, they are demanding an extra ₹3 lakh claiming unforeseen development costs or they won't hand over the keys. I've already stretched my savings to the limit and feel trapped. Is this demand legal? What actions can I take against the builder for this extortion? I am really stressed as my family is supposed to move in next month.