call a method in code-behind from client side script

See more:

C#

ASP.NET

Javascript

Ajax

Hi,
I'm using asp.net.
I want to update a part of my page (not whole page) without sending the page request to the server. I want to do this in the client side.
for this, I think I should use AJAX and javascript.

I've created a handler page: handler.ashx
I send a request and get response in plain text:
(in javascript)
...
XMLHttpRequest.open("GET", url);
XMLHttpRequest.onreadystate = ApplyUpdate; // ApplyUpdate is a function that get the response in client side(javascript).
XMLHttpRequest.send(null);
...

(in handler.ashx)
...
response.write("plain text as response");
...

Now in ApplyUpdate function I can use the plain text and show it in client browser.

BUT
the problem is this:
I have a placeholder1 control in asp.net: <asp:PlaceHolder ID="PlaceHolder1" runat="server">

How can I assign the plain text (the response !) to a label control and add the label to placeholder1 control !?
I mean how I can use these codes in client side:
Label lb1 = new Lable();
lb1.text = plaintext;
placeholder1.controls.add(lb1);

these codes are in c# (code-behind) but how I can use such these codes?
I cannot do the same thing by javascript codes !

excuse my bad explanation.
I'll appreciate any help.