How To Loose My Job- Employer Refuses To Release Even After Serving The Notice Period

Now my employer refuses to re-leave me after completing 23 days notice period.But my new employerSaid above I must join on 16 August to my new company.
In this case what to do?
I have resignation accepted copy,copy of mail asking acknowledgment letter to HR.
What legal action shall I take against them?

Dear Sachin,
If you have received an acceptance of your resignation, it puts your process in progress. However, please mail the employer, offering any kind of buy-out, if applicable. Make sure you put an end date to your release. Complete all your formalities, including the knowledge transfer and financial declaration respectively. Review this clearance process to avoid every loose end. File the documents, declaring closures from the respective departments. Maintain hard copies of all these communications. Remain polite but firm.
Finally, you may consider to declare that, in case, no reply is received from them post the due date, it would be taken as a clearance.