Facing Unpaid Overtime and Threats of Withheld Salary: What Are My Legal Rights?

I joined an event company as a designer two months ago and am currently in a probationary period for six months. During my interview, I informed HR that I would not be able to provide overtime beyond my working hours due to certain conditions at home. Despite this, I am regularly required to work overtime for 1.5 to 3 hours without any extra payment. The company policy states that employee working hours are from 10 AM to 6:30 PM, but my shift often extends beyond these hours without additional pay.

I have informed HR that I cannot continue under these conditions and have offered my resignation. However, the management has threatened to withhold my entire December salary if I resign without serving a notice period. Can they legally do this? What legal action can I take if my salary is withheld?

Thanks in advance.

I should also mention that I received an official list containing my company's policies, and the foremost point was that all employee timings are from 10 AM to 6:30 PM, Monday to Friday, with a grace period of 10 minutes. However, almost every day, I worked overtime until 8:15 PM, 8:45 PM, and even 9:30 PM without any extra pay. I was supposed to show up at the office at the mentioned time and not be late. This is the reason I had to take 5-6 leaves since my duties at home were getting ignored, and thus, I have decided to resign from this office. Now the company is threatening to withhold my salary for which I have already worked if I don't work for the notice period mentioned in the offer letter.