Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(beginReq);
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(endReq);

var btn;

function beginReq(sender, args) {
    btn = args.get_postBackElement();
    btn.className = "hide";
}

function endReq(sender, args) {
    btn.className = "show";
}