My company is forcing me to sign Non Compete agreement

Hi, I have been working in a startup company since last 16 months. I have now quit the company. They have not paid my last one and half months salary. They are saying unless I sign the non-compete agreement, they will not release my salary nor experience letter. The non-compete agreement says I am not supposed to work in any similar businesses of the startup or its competitors, nor start one on my own. It doesn't mention any location constraint. Time constraint is 12 months from the date of termination. Can such a law be enforced in India ? And is it permissible for them to hold my salary and force me to sign ? However I had signed offer letter while joining, which mentions the same clause. But then the company was not registered. Company is registered now. (If that matters). If at all I start my own business in the same field (without causing any damage to them nor dealing with any of their clients or customers), will they be able to sue me ? PS: They emailed me the non compete agreement, asked me to print it, sign and courier it to them.