Q: browser has been blocked by FBI

It's malware ...

Download and run the adware removal tool here >   The Safe Mac » Adware Removal Guide

Easy, safe, and only takes a minute or two.

Once you have removed the malware, quit and relaunch Safari to test.

It's not malware. It's a JavaScript scam that only affects your web browser, and only temporarily.

1. Some of those scam pages can be dismissed very easily. Press command-W to close the tab or window. A huge box will pop up. Press the returnkey and both the box and the page will close. If that doesn't happen, continue.

2. From the Safari menu bar, select

          Safari ▹ Preferences... ▹ Security

and uncheck the box marked Enable JavaScript. Leave the preferences dialog open.

Close the malicious window or tab.

Re-enable JavaScript and close the preferences dialog.

3. If the Preferences menu item is grayed out, quit Safari. Force quit if necessary. Relaunch it by holding down the shift key and clicking its icon in the Dock. From the menu bar, select

          Safari ▹ Preferences... ▹ Privacy ▹ Remove All Website Data

to get rid of any cookies or other data left by the server. Open your Downloads folder and delete anything you don't recognize.

Go to Step 3.

Thank you so much for posting - this happened to me too with a National Security warning - which seemed scary but I knew it was a scam.  I had to do step 3.  Thank you again.

I am having the same problem but all three options haven't helped me.. Any other ideas!?

I am at my wits end.   

I had the same problem and i called support -1-855-766-9909 and got the support.

How do you do this on iPhone?

Similar to above:

Settings->Safari->Advanced then turn off JavaScript

go back into the browser and click cancel/ok through a few more of the prompts. It eventually resets itself and stops working.

Cloise the website as well as the one that got you there.

Then go back and turn on JavaScript.

One other caveat, there seems to be other websites willing to 'help' remove this from your computer,they will state that even though your browser is unlocked the program is still running inside and will eventually slow down and disable the system. Blogs such as You Care offer 'support' to help you get rid of this so called virus. I suggest relying on help from Apple support groups such as this one.

this worked for my ipad........home button>settings>safari>clear history & website data>home button>relaunch safari