use behaviorId of modal popup extender
cc1:ModalPopupExtender ID="mdllogin" BehaviorId="modal1" TargetControlID="linklogin" PopupControlID="popUpPanel" BackgroundCssClass="modalBackground" CancelControlID="btnclose" runat="server">
</cc1:ModalPopupExtender>
public void login()
{
modal1.Show();
}