First, I would suggest you remove your name, mobile number, and contact information from the letter and repost it. It is dangerous to share those as unscrupulous people and bots can pick them up.
Legal Implications of Signing the Letter
You have made a mistake in signing such a (sorry to say) unwise letter and landing yourself in hot water. However, Indian law does not allow contracts to restrain trade, so the letter is invalid and void ab initio (from the beginning). Further, the letter specifically mentions the employer-employee relationship, which begins from the appointment letter and not the offer letter.
Preparing for Legal Action
These are matters to be discussed in court, which can be an expensive affair, so you need to be prepared for it. Let them try to recover this in court; it will be far more expensive for them than they think they can recover. In the meantime, go and speak to an advocate, have him frame a response, and send a cease and desist letter on grounds of harassment, defamation, and any other grounds he can think of.
Seeking External Support
Also, go and meet the labor commissioner of your area, any pre-labor NGO/activist organization in your area, and maybe local political parties, all those who can make life miserable for them.