Popup show and hide from both server side and client side

I have a popup div to which i am showing that from server side. I want to hiding that popup from client side. Any suggestion please feel free to share.
My code is given below:-

design page
------------------------
<div id="Popupdiv" runat="server" class="popupwindow" visible="false">
<img alt="" src="../images/close.png" onclick="f1();" />
</div>

javascript
--------------------------
function f1()
{
div1 = document.getElementById('<%=Popupdiv.ClientID%>')
div1.style.visibility="hidden";
}

code behind
------------------------------------------
Popupdiv.Visible = true;


It works fine but measure problem is for every post back that popup div displying automatically.