var prm = Sys.WebForms.PageRequestManager.getInstance();
prm.add_initializeRequest(InitializeRequest);

function InitializeRequest(sender, args)
{
    if (prm.get_isInAsyncPostBack())
    {
        args.set_cancel(true);
        
        alert("A request is currently being processed," +
            "\nPlease wait few seconds before refreshing.");
    }

    function AbortPostBack()
    {
	    if(prm.get_isInAsyncPostBack())
	    {
		    prm.abortPostBack();
	    }
    }
}