(Reuters) - Twitter, the popular Internet micro-blogging service, said it is fending off a cyber attack that prevented people from accessing its website for several hours on Thursday.

Twitter co-founder Biz Stone said on Twitter's blog that the site was the victim of a denial-of-service attack, a technique in which hackers overwhelm a website's servers with communications requests.

"We are defending against this attack now and will continue to update our status blog as we continue to defend and later investigate," Stone wrote.

Resource Library:

A separate Twitter status Web page said "the site is back up, but we continuing to defend against and recover from this attack." The time stamp was listed as "1 hour ago", as of 12:20 PM Eastern Time.

The Twitter site remains inaccessible. Some people also reported delays logging in or posting to their Facebook profiles.

A spokeswoman at the popular social networking site did not immediately respond to a request for comment.

Asked for comments on the outage in an email, Twitter's Stone referred to the blog posts.

Twitter, which lets users publish short, 140-character messages to groups of online "followers," is one of the fastest-growing Internet companies.

The number of worldwide unique visitors to the Twitter website reached 44.5 million in June, up 15-fold year-over-year, according to comScore data.

In the U.S., Twitter broke into the top 50 web properties by traffic for the first time in June.

(Reporting by Alexei Oreskovic, editing by Gerald E. McCormick)