Present employer not relieving me

Presently I working in Ltd company and I am in probation period (according to clause in my appointment letter there is no need to serve any notice period in probation )I have given official resignation letter bacause I got a new job opportunity which I think right for me and my resignation is accepted by HOD but HR not ready to relive me and they forces me to serve notice period of one month there after they will relieve me with relieving letter and with settlement but now I am mentally harassed and not prepared to do the job kindly tell me what should I do?