Client-Side Event to disable button immediately.

<script language="javascript">
var c=0;
function DisableClick()
{
var objName = 'Button1';
document.getElementById(objName).disabled=true;
c=c+1;
msg = 'Please Wait...('+ c +')!';
document.getElementById(objName).value= msg;
var t=setTimeout('DisableClick()',1000);
}
</script>

And following button code in body.
Button Control – HTML Button instead of Asp button.

<input id="Button1" name="Button1" onclick="DisableClick();" onserverclick="Button1_Click" runat="server" type="button" value="Submit Payment" />

Note: onclick event is calling javascript client-side function and onserverclick event is calling server-side function which performs actual task.

Server-Side Event to perform actual task
This task can be any as per your logic, for an example I am performing some heavy time consuming task,

you can even make use of Threading concept to minimize code…

protected void Button1_Click(object sender, EventArgs e)
{
ArrayList a = new ArrayList();
for (int i = 0; i < 10000; i++)
{
for (int j = 0; j < 1000; j++)
{
a.Add(i.ToString() + j.ToString());
}
}
Response.Write("I am done: " +
DateTime.Now.ToLongTimeString());
}

Before Button Click