Police

Generally, the term "arrest" in it's ordinary sense, means the apprehension or restraint or the deprivation of one's personal liberty. Therefore, only the deprivation of liberty by legal authority or at least by apparent legal authority, in a professionally competent and adept manner amounts to arrest.

However, there are certain rights that are provided to every person who is arrested by the Cod of Criminal Procedure i.e. CrPC (1973), these are as follows -
a) Right To Silence,

b) Right To Know the Grounds of Arrest,

c) Information Regarding the Right to be Released on Bail,

d) Right To be Presented Before Magistrate Without Delay,

e) Rights at Trial [which includes Right to a Fair Trial & Right to a Speedy Trial],

f) Right To Consult a Legal Practitioner,

g) Right of Free Legal Aid and

h) Right To be Examined by Medical Practitioner.

As per the above given rights, you have a Right to know the Grounds you're arrested for. This Right has been offered by various sections and Articles of CrPc and Constitution respectively.
Down below are given all the sections and articles that provide this Right to the every person being arrested.

According to Section 50(1) of CrPc, every person who is being arrested by any police officer, without any warrant, is entitled to know the full particulars of the offence for which he or she is being arrested and that the police officer is Duty Bound to tell the accused such particulars and cannot deny it.

As per Section 55 of CrPc, when any person is being arrested by any police officer, who is deputed by a senior police officer, then such subordinate officer shall before making arrest, notify the person to be arrested the substance of the written order given by the senior police officer specifying the offence or other cause for which the arrest is to be made.

Moreover, Section 75 of CrPc states that if a person is being arrested under warrant then any person executing such warrant must notify the person to be arrested the particulars of such warrant. In fact, the Constitution of India also confers this right as one of the Fundamental Rights under Article 22(2).

Yes, you may file a defamation case against the police officers for defamation if you're found guilty. This has already took place in a renowned case of PRASHANT KANOJIA Vs STATE Of U.P. Thru. SENIOR SUPERINTENDENT Of POLICE, LUCKNOW.

Down below is the link for the Judgment passed by the Apex Court (SUPREME COURT OF INDIA) in the above said matter to which you may take a look for reference.

https://indiankanoon.org/doc/96887226/