Custody of Children

Q: I had 3 children from a previous marriage. I thereafter got married as a 2nd wife. My children went to their father for the holidays and now when it is time for them to come back, their father refuses to send them. He says they will live with him now and that shariah is on his side because I got married to another man. I am so distraught I don't know what to do. I love my children and I don't want to lose them. Please tell me what my rights are. Jazakallah