Something

default.aspx:
<%@ Page language="c#" Culture="en-US" CodeFile="default.aspx.cs" Inherits="DefaultPage" %>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" >
<html>
        <head>
                <title>UTF8 Test</title>
        </head>
        <body>
                <form id="Form1" method="post" runat="server">
                </form>
        </body>
</html>

default.aspx.cs:
using System;
using System.Web;
using System.Web.UI;

public partial class DefaultPage: Page
{
    void Page_Load (object sender, EventArgs e)
    {
        Response.Write("UTF8 string: <strong>&#321;ó&#378;&#380;&#281;&#347;&#263;</strong>");
    }
}