Company denying to issue relieving letter

Dear Sir/Madam, Myself Jeevan Ankam from Hyderabad, working as sales manager. recently i got an opportunity with a beg company and sames informed to my present company. verbally management accepted the same and asked me to handover all customer details to company. after my present company confirmation i have accepted my new company offer. even i got and email from my present company as acceptance of my resignation (Date not mentioned). now my present company wants me to serve 30 Days notice period. in the offer letter of my present company they haven't mentioned 30 Days Notice period. now they want me to serve till 30 days after this period they will release the reliving letter or they will release the reliving letter and salary after 30 Days from the date of reliving. request you to advice me on this present situation. Thanks & Regards Jeevan Ankam