Redirect a page after viewing alertmessage

ScriptManager.RegisterClientScriptBlock(this, typeof(Page), "Hi", "alert('This is popUpMessage');", true);

OR 

CleintScript.RegisterStartupScript(this.GetType(),"key","<script type='text/javascript'>alert("This is popUpMessage")</script>")
Response.redirect("loginpage.aspx");

<asp:Button ID="btnSubmit" Text="Submit" runat="server" />
<cc1:ModalPopupExtender ID="mpe" runat="server" TargetControlID="btnSubmit"
    PopupControlID="pnlPopup" BackgroundCssClass="modalBackground">
</cc1:ModalPopupExtender>

<asp:Panel ID="pnlPopup" runat="server" >
    <table width="100%" >
        
        <tr>
            <td>
            </td>
            <td align="right" style="padding-right: 15px">
                <asp:UpdatePanel ID="upCancel" runat="server" UpdateMode="Always">
                    <ContentTemplate>
                        <asp:Button ID="btnYes" OnClick="btnYes_Click" runat="server"/>
                    </ContentTemplate>
                    <Triggers>
                        <asp:AsyncPostBackTrigger ControlID="btnCancelYes" EventName="Click" />
                    </Triggers>
                </asp:UpdatePanel>
            </td>
        </tr>
    </table>
</asp:Panel>

protected void btnYes_Click(object sender, EventArgs e)
{
           mpe.Hide();
           //submit for data into database
          Responce.Redirect("Login.aspx",true);
}

Page.ClientScript.RegisterStartupScript(this.GetType(), "RegSucessScript", "RegSucessfully();", true);

function RegSucessfully() { 
alert("Registered Sucessfully");
this.window.location.href = 'YourRedirectPage.aspx'
}