I went through a situation wherein the police officer arrested a person without giving any reason and has kept in custody for a day. How to deal with it?

Answer: There are multiple occasions where such instances are faced by public at the hands of law enforcement agencies. The Honorable Supreme Court (D.K.Basu case) as well as National Human Rights Commission of India has laid down guidelines to be followed by the Police in case of arrest of a person with specific pre-arrest and post-arrest norms.
The above question involves absolute illegality on the part of police and it should be reported to the higher authorities as well as to the State Police Complaints Authority so also to the State Human Rights Commission as the Police Officer is in contempt of the Supreme Court and in violation of the NHRC guidelines.