$(document).ready(
            function () {
                Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(onRequestStart)
                Sys.WebForms.PageRequestManager.getInstance().add_endRequest(onRequestEnd)
            }
        );

function onRequestStart() {
    $.blockUI({ message: '<h1>Wczytywanie...<h1>', css: { color: '#CCCCCC', border: '0px none #fff', background: 'Transparent'} });
}

function onRequestEnd() {
    $.unblockUI();
}