Coolite 

        <script type="text/javascript">
            function DoWork(panel){
                var loadMask=new Ext.LoadMask(panel.body, {msg:'Working 5 sec...'});
                loadMask.show();   
                
                //emulate 5 sec work
                var task = new Ext.util.DelayedTask(function(){loadMask.hide();});      
                task.delay(5000);            
            }
        </script>
        <ext:Panel runat="server" ID="Panel1" Title="LoadMask processing example" Width="300" Height="150">
            <Content>
                Content
            </Content>
            <Buttons>
                <ext:Button runat="server" ID="SomeWork" Text="Do work" AutoPostBack="false">
                    <Listeners>
                        <Click Handler="DoWork({Panel1});" />
                    </Listeners>
                </ext:Button>
            </Buttons>
        </ext:Panel>