Her parents can file a false case of kidnapping against you. However, as both of you have attained the age of majority there is no legal action which the police can take against you if the girl gives a statement to the police that she has come out with you with her own free will.

As it is common knowledge that police in India ranks amongst the most corrupt government departments the possibility of cops colluding with the parents of your girl friend cannot outright be rejected. In order to protect your rights you may move the HC to seek protection from any kind of harassment by the police and/or the parents of your lover. If such an order is passed by the HC the police will not even think of violating your legal rights.

Go as per above advise.

approach high court get protection order or u can get marriage & get it registered in front of registrar

What I suggest is that you should try and talk to the parents, if they are not agreeable to the same then you can approach High Court with directions to the Police for protecting you and your girlfriend.

The HC protection order would be a declaration of your legal rights as a major. Moreover, it will be a loud and clear message to the police to follow the law of the land and honour your rights.