each time I go with the mouse over a website or click a link, Safari opens a spam website which may open other websites automatically.

It is very annoying. I deleted all cookies, deactivate all plugins and extensions but the problems persist.

Anyone experienced the same and got a solution?

Here a screen shot of an unwanted thai website which opens most of the time.

Your computer may have downloaded a java-based virus or malware.

I would suggest visiting a trusted MAC forum and following instructions for detecting and cleaning.

You could also try resetting Safari.

I did reset Safari. It made it faster, however the problem became worse. Strange thai websides open automatically and require me to confirm something.

I am using in the meantime Firefox without any problems.

Try Onyx. It's a good maintenance utility. It also does cleaning.

I run all the 'Automation' scripts, periodically.

http://www.titanium.free.fr

Sounds like you may have been duped into inadvertently installing some sort of malicious adware. Resetting Safari will not remove adware, but this will:

http://www.thesafemac.com/art/

Hi thanks for all suggestions. I did not try them yet.

I found one culprit which was responsible for all those underlining links on the website. It is 'Nav-Links'. Since I disabled it, Safari is almost back to normal.

For other users: Don't install this extension!

Yes, Nav-Links is indeed adware and might create the problem you were having. You should probably still download the adware removal tool from the link I posted earlier and run it. You may find other culprits. It's amazing how many savvy users have been duped into installing adware. Genio is one example. I ran the Tsmart script and found it on two of my machines. No idea how it got there!