Avoid button clicking before the post back finish ??

I have asp.net page with reportviewer control, few textboxes and and a button.


On the button click the page will display the reportviewer with the generated report.


This wil take few moments to generate the report , depend the record counts.


Sometimes user click the button and when its take few moments to load the report the user click the button again and this report error from the server.


 I want prevent the user click the button again after the first click .
 
Screenshot before post back 
Screen after post back 

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