ASP.NET4 C#: Adding OnClick event to Panel control

Dear DotNet dudes
I'm having a lot of trouble achieving something which I'm sure must be simpler than I'm making it.
I have a client questionnaire form that is split into 3 sections. Each section is a Panel control (pnlSection1, pnlSection2 & pnlSection3).
I only want 1 Panel to be enabled at any time.
The user will click on a panel to enable it (this will also disable the current panel). 
Clicking on a panel will also change the CssClass from "cssDisabledPanel" to "cssEnabledPanel".
I only want the Panel OnClick event to fire when clicking on a disabled Panel. I do not want a postback when clicking on the enabled Panel.
Unfortunately, the Panel control has no Onclick event.
Below is the markup showing what I want to achieve:
<form id="form1" runat="server">     <div>         <asp:U ...