Company Not Accepting My Relieving And Not Giving My Acedamic Certificates Back What To Do?

Hi 4 years back, I have joined a software company. The Employer took my academic certificates and a bank cheque for safety deposit purpose. Employer took my agreement on a bond for 42 months. At that time employer didn’t gave me appointment letter or offer letter. When I asked about appointment letter, replied like “As we are a small company, we will give you offer letter and other documents at the time of relieving”. 6 months back, my bond agreement period was completed with my employer. So first I requested for the documents in this process. Employer gave me appointment letter and pay slips only and he rejected to give my certificates and cheque, he is saying that they will give me all those remaining documents at time of relieving. I asked about the notice period he said it was 3 months. Now I got the opportunity with other employer, so I want to relieve from my current employer. I have applied for resignation on 18th February 2019 with the last date as 18th May 2019, but my current employer not accepting my resignation and not willing to issue my relieving letter. And he was saying that our client was not accepting your resignation. The clauses in appointment letter are as follows: 1. Terms of Employment: Your employment shall continue on an indefinite basis, subject to termination by either by yourself or by the Company as per any of the following criteria: (a) If you are in the middle of the project and responsible to perform key activities, the company has the right to increase your notice period until such time that the above activities are completed/handed over to another designated employ of the company. (b) In the event of the employee’s committing breach/default of any of the terms and conditions contained herein above, the employee shall, within a week after committing such breach pay to the employer caused by the damage. They didn’t mention the notice period duration in the appointment letter. I don’t want to continue in this company. Is client having any right to hold me in the company? Is it necessary to get the relieving letter from my current employer? Is my current employer can have the right to increase my notice period? If then how much they can increase it? What is the maximum notice period as per the employment law in India? Please suggest me possible ways to get out of my current employer.