Certificates are not giving in the Company

I am vijay, working as a software engineer in a software company which is located in hyderabad. i had given 1 year bond to work, but they added as 3 months probation period and 12 months service period. i joined 19 march 2014, i completed by bond period on 30 june 2015. before joining i had given my academic certificates, previous working company experience, relieving letters and pay slips. i got a job in an mnc company. as per company norms, notice period is 2 months. i given my resignation letter on 18 july 2015. now, they are not considering my resignation letter as well as forcing me to work one more year. other wise, they told me that they won't return my certificates. how can i get my academic certificates, previous company experience, relieving letters, pay slips and also, here in this company i worked for an year. i need this company experience, relieving and pay slips. i have the company offer letter and bank account statement (salary paid every month). i requested so many times, but they are not considering my resignation. can anybody give me some idea,how to resolve this issue?