Show Progress Img Over Gridview

I want to show a progress img when gridview is performing some operation. When i click on update linkbtn of gridview i have written some code in code-behind which takes some time Meanwhile i want to show progress img over gridview to notify user that some operation is being performed. Progress img should cover the full size of gridview. How can achieve this?

Sample code:

<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<div style="background-color:Gray">
<div>
<asp:GridView ID="GridView1" runat="server">
</asp:GridView>
</div>
<div style="position:absolute;">
<asp:UpdateProgress ID="UpdateProgress1" runat="server">
<ProgressTemplate>
<img id="imgProgress" src="loading.gif" />
</ProgressTemplate>
</asp:UpdateProgress>
</div>
</div>
<asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" />
</ContentTemplate>
</asp:UpdatePanel>