Can my employer fire me for refusing to work extra hours without pay?

I'm working at an IT company in Hyderabad and my employer is forcing us to work extra hours almost every day, but there's no overtime pay. When I refused last week, they threatened to terminate my employment. I'm really worried because I have a family to support. I've been with this company for 3 years and never faced such issues before. Is this legal, and what can I do to protect my job and rights?