What to do if company delays salary and documents?

I am a software engineer (non-managerial) working for the past 9 months with a salary of 7.5 LPA. My appointment letter mentions that I am required to stay with the company for at least 18 months after confirming my full-time employment. There is no mention of any compensation if I breach this clause. I want to resign and serve a full 3-month notice period, but the company is saying that they will not allow me to leave even after serving the 3-month notice period. I want the relieving documents and experience letter from them. When I asked about bond compensation, they said they do not take money. They have also been delaying my salary by 10-20 days for the last 4 months and have not paid the variable components for the last two quarters.