Denial for relieving letter

Hello Sir, I am an Sr. engineer in an IT Support Pvt.Ltd. company and resigned from my position after serving entire 60 days notice period after 10+ yrs of service tenure, where my resignation was accepted and on the last day company hold my exit process and asked me not to join OEM company. My exit formalities and relieving letter is kept on hold by the company. Company says there is an agreement with OEM, under which if I join OEM then the agreement will be breached between my old company and OEM. Hence they have kept my exit on hold and asking me to give written confirmation that I will not join OEM at least for a year which they will make sure and then only issue me my relieving / exp letter with final settlement. My question are below, 1. if I have completely served the notice period and my resignation is accepted then, does company can hold my relieving letter along with full and final settlement process? 2. I confirmed with OEM HR on mail that is there any issue for my employment at OEM company in future, as I'm being the partner ex-employee, and they told no issues are there, just give us relieving letter within 45 days or they can not continue my service without relieving letter. (This proves that there is no such agreement and my company is only threatening me and holding my relieving process) 3. How valid is the relieving letter? as I have all salary slips, form 16, a/c stmt, appraisal letters for 10+ yrs 4 Please suggest what are my basic rights and what should be done to get my relieving letter as I really wish to join OEM which is a huge opportunity for me. Thanking you in advance