Security - is my website under attack?

A few days ago, I enabled error logging in CodeIgniter. It generates a log file per day. Analysis shows that there are a lot of page requests to non-existing pages on my domain, on and on. Sometimes just a few seconds after one another.
Is there a reason to be worried?
How can I detect from which IP-address these requests are done? I would like to ban that IP-address in .htaccess (deny).