protected void btnSubmit_Click(object sender, EventArgs e)
        {
             ErrorMessage(message);
        }
       protected void ErrorMessage(string message)
        {
            ScriptManager1.RegisterDataItem(lblError2, message);
        }

    <div id="divError" class="errorBox">
        <asp:Image ID="imgWarning2" ImageUrl="~/Images/warning.png" runat="server" Visible="False" />
        <asp:Label ID="lblError2" runat="server" Text=""></asp:Label>
    </div>
        <div class="bottomButtons">
            <asp:UpdatePanel ID="upnlPlaceOrder" runat="server">
                <ContentTemplate>
                     asp:Button ID="btnSubmit" runat="server" CssClass="iButton" Text="Submit"
                          OnClick="btnSubmit_Click" />
                </ContentTemplate>
            </asp:UpdatePanel>
        </div>
    <script type="text/javascript" language="javascript">
        var prm = Sys.WebForms.PageRequestManager.getInstance();
        prm.add_pageLoaded(UpdateLabelHandler);

        function UpdateLabelHandler(sender, args) {
            var ControldataItems = args.get_dataItems();
            if ($get("<%=lblError2.ClientID %>") !== null)
                $get("<%=lblError2.ClientID %>").innerHTML = ControldataItems["<%=lblError2.ClientID %>"];
        }