This is a very sensitive situation. As per law, the letter you signed before the Panchayat is invalid and does not have any legal sanctity. It is a waste paper in the eye of law. The demand of another girl and her parents is quite reasonable. You better contact me through Kaanoon.com.
If you want your ex-wife to agree for mutual consent divorce, you have to file a case of Bigamy against the girl and her parents under 494 IPC. Marrying another person having legally wedded husband is very serious section having 7 years punishment. To avoid this punishment they will come to you and agree for mutual divorce.
If the panchayat people threatening you not to file any case, you can file a case under IPC and Human Rights Commission. Police will arrest all the panchayat people. But I wont advise you this step. Because all the community people will become against you. You just warn the community people that if they come in between your personal matrimonial matters, you will file the above cases. Inspite of that they interfere into your personal matters, then file the above cases, there is no other way.