How to authenticate user while calling WCF service using AJAX?

I have a WCF service which needs to be called directly from the browser. I am using ScriptManager on ASPX page to add a ServiceReference to the WCF service. I am able to call service methods from JavaScript and get the results back.  But I want to deny anonymous users accessing the WCF service. Is there any way to do user authentication before calling a service method from JavaScript?
 

...