Employer denied to provide relieving letter

Hello sir/mam, I request your kind advice for my issue to get resolved. I worked for a company around 4 months, due to pathetic environment and poor management I decided to change my company. After got offer from another company I wrote a resignation mail to HR and he accepted the resignation in reply. It was a normal exit. Then I was told by him that I'll get my relieving letter by 25-30 days. But now he avoiding my mails and calls. Even founder of company straightly denied to provide relieving letter. Now I need that letter to be submitted to currently joined company. And it's my right to get offer letter and employer can't deny that. I need your advice in this matter. Please do the needful. It'll be great help. Thanks!