What is considered a legal marriage

I am a resident of Bangalore and my father is in Delhi. I want to marry a girl who was my college classmate. We have been seeing each other for the last 7 years while we know each other for last 10 years. For the last 5 years we have been staying together and shifted 3 places of residence in Bangalore. We have been staying together without marriage but most of the time the landlord and the neighbours have assumed that we are married. I informed my parents that I want to marry this girl a couple of years back. My mother passed away last year and my father is not ready for the marriage. I did not inform them that we stay together. However, the girl informed her parents about her intention to marry me a couple of weeks back. Her parents came down to Bangalore without informing her to her friend's residence where she had told them she was staying. She said she had shifted to stay with some other friend but they got a reasonable doubt that she was staying with me. They have taken a separate home in Bangalore and staying with her now. She has a brother as well. Their place of residence is Rohtak, Haryana. Now her parents are controlling her free will, following her to work, cursing her, threatening to kill her and me if she tries to speak to me or see me. I spoke to her on phone and she is in depression, crying continuously. They are forcing her to do what they want. She is 28, I think she has a right to choose how she wants to live. They are even saying that they will make her quit her job and keep her confined at her home in Rohtak so that she cannot meet anyone. My age is 29. Sorry for the long build up, but I was looking for help here. We are confused if we can get married outside with minimum fanfare but ensuring legality so that we are legally safe. I read about court marriage but some people mention that some lawyers fluke people and court marriages are not really legal. If you could please outline the complete legal procedure for getting into a valid marriage which cannot be challenged or threatened by police, it will be very helpful given the scenario we are stuck in and trying to live together safely.