Hackers have accessed the accounts of a series of celebrities on the popular micro-blogging site Twitter and left a variety of pornographic and dubious messages and links.

In the most serious breach of security for the burgeoning blogging site, beloved of the tech community, hackers have targeted more than 30 accounts including those belonging to Britney Spears, Barack Obama, Facebook, CNN anchor Rick Sanchez and the Huffington Post.

Twitter users post short updates - of less than 140 characters - on their accounts which can be followed by other users. The site has rapidly gained popularity since its launch in April last year.

Increasingly celebrities have signed up as a way of connecting with their fans and many news outlets use Twitter to alert people to new stories posted on their websites.

The hackers found a way to take over the online tools used by Twitter's support team to post the false updates or tweets, Twitter said in a blog posting.

During the security breech Britney Spears falsely posted about the size of her genitals, Barack Obama (who has not used Twitter since his election victory) posted a link to a poll with the prize of winning free petrol, Rick Sanchez said he was not coming into the office because he was high on crack cocaine and Facebook listed a link to a pornography site. All the false postings have now been taken down and the accounts returned to the rightful owners, Twitter said.

The attack, described by Twitter as "Monday morning madness", came after a weekend of phishing attacks on the site which fooled thousands of Twitter users into revealing their details. Twitter said the breaches of security were not linked.

In the blog posting Twitter said: "This morning we discovered 33 Twitter accounts had been "hacked" including prominent Twitter-ers like Rick Sanchez and Barack Obama. We immediately locked down the accounts and investigated the issue. Rick, Barack and others are now back in control of their accounts.

"The issue with these 33 accounts is different from the Phishing scam aimed at Twitter users this weekend. These accounts were compromised by an individual who hacked into some of the tools our support team uses to help people do things like edit the e-mail address associated with their Twitter account when they can't remember or get stuck.

"We considered this a very serious breach of security and immediately took the support tools offline. We'll put them back only when they're safe and secure."

The hacking attack comes at a bad time for Twitter which is poised to break out of the tech community to become a mainstream social networking tool. Ev Williams, one of the site's founders, tweeted that the atmosphere at the office today was "focused anxiety".