Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(beginRequest);
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(endRequest);


function beginRequest(sender, args) {
    for (var f = 0; f < document.forms.length; f++) {
        var frm = document.forms[f];


        var mustSubmit = true;
        if (frm.onsubmit != null)
            mustSubmit = frm.onsubmit();

        var els = frm.elements;
        for (var i = 0; i < els.length; i++) {
            if (els[i].type == "submit")
                if (mustSubmit)
                    els[i].disabled = true;
        }
        return mustSubmit;

    }

}
function endRequest(sender, args) {
    for (var f = 0; f < document.forms.length; f++) {
        var frm = document.forms[f];


        var mustSubmit = true;
        if (frm.onsubmit != null)
            mustSubmit = frm.onsubmit();

        var els = frm.elements;
        for (var i = 0; i < els.length; i++) {
            if (els[i].type == "submit")
                if (mustSubmit)
                    els[i].disabled = false;
        }
        return mustSubmit;

    }

}