Employer has kept my originaleducational document

Hi Sir/Ma'am I recently joined an organization in Noida and I was asked to submit my original marksheet. When I asked them they said it was for police verification and that they will return after 3 months once the police verification is done. So I submitted my document but I did say that I would need my document for some days before three months time period as I would be taking admission for further studies. And the HR said that I can take it back for some days during the admission for my studies. But after I completed a week in the organization i did not like the working environment as I observed that many employee were forced to do overtime as they can't even say anything as their documents were also taken as a deposited. And that's when I Googled about this practice and found that many are of the opinion that this practice is illegal. So next day I went to office and wanted this to be clarified I was treated rudely by the employer of the company. So I decided not to work with them and asked them to return my original marksheet that I had submitted. But the employer said they can't release my document and I am bound by contract to serve in the organization for 6 months. And if I leave before six months I will have to give them a month's salary as well as serve them notice period. Please advise how can I get my original documents back from the employer and what legal action can I take. I am unable to visit the state board to get the duplicate at this time. I have already left that organization. Your advice will be appreciated. Thanks.