Software Company Denying To Relieve Me Even After Notice Period Is Served.

Hi, I am working as a software engineer in an organisation in noida. I served my notice period of two months but company is forcing me to complete a task which is of not just one or two days although I tried it and extended few days further. They are saying to finish the task then only they will give me experience letter and my notice period salary. I need to join a new organisation. I am really stuck and see no way out. Please suggest. Thanks in advance.