Can i file a mental harassment case without notice or fir against a group of locals

If you are offended then you may have to lodge a criminal complaint with the police seeking protection and proper legal action against the offenders.

You cannot approach court with a private criminal complaint without exhausting the remedy available with the local police in such law and order problem.

I agree with the opinion and advise of learned expert Mr. T Kalaiselvan.

File police complaint and move to Magistrate if no action is taken.

but the problem is police is generally reluctant to write a complaint  against one person ,there is a very rare chance that they will take complaint group of locals and also this group will counter accusations to avoid any action .. so that is the major problem

You have to exhaust the remedy before police first and when it is not fruitful then you can approach court of law for remedy