Hundreds of websites have been defaced by hackers as the row over cartoons mocking Mohammed moves online.

Internet monitoring group Zone-H has reported more than 500 attacks by hackers leaving slogans threatening retribution for publishing the cartoons that have sparked anger all over the world. 

"Several hacker groups from different Muslim nations united their forces in order to produce the large amount of damage on Danish and, more generally, western web servers," said Zone-H.

"During the attacks they promoted both moderate and extremist manifestos through the defacement of the homepages, promoting a boycott campaign throughout the digital Ummah against Danish products."

The furore has brought at least one hacker out of retirement. Zone-H reported that a hacker called 'DarkblooD' dropped out a year ago but has resumed his activities with a message of protest to Per Stig Møller, the Danish minister for foreign affairs.

"The news and the cartoons were horrifying and extremely disturbing to me, and I believe that all Muslims who read, viewed or learned about this news were equally saddened, disappointed and disturbed," he wrote.

"All criticised such work and felt awful and dismayed about it. Similarly, I believe that all sane and wise people would feel the same about it."

Security firm F-Secure has hosted pictures of the defacements on its website here.