Wikipedia users were greeted by a swastika flag when a National Socialist activist managed to set the image as the template background for several pages on the online encyclopedia.
"An account modified a Wiki template for English-language Wikipedia, resulting in a Nazi flag appearing for a few minutes on a number of articles covering a diverse range of topics," a Wikimedia spokesperson confirmed.
Jewish and Antifa social media users were baffled by the action, with one asking: "Did Wikipedia get hacked?! Why is there a Nazi flag on the main page?" while another said: "Why is there a Nazi swastika when I look up Steven Pinker on Wikipedia?"
Wikipedia is owned by the Wikimedia Foundation, a US-based organization that has annual revenues of more than $125m (£90m) and more than $180m (£130m) in net assets which has been criticised for its fundraising tactics.
According to the Wikimedia spokesperson, "the vandalism featuring the Nazi flag was reverted by Wikipedia volunteers within five minutes" and administrators added additional "protections" to the template within 15 minutes.
The spokesperson added: "This type of behaviour is unacceptable on Wikipedia and violates a number of Wikipedia's policies."
"Volunteer administrators have already fixed the vandalism, blocked the account responsible, and will further evaluate the situation to see if additional recourse is needed. This particularly vile action - a form of vandalism on Wikipedia - is something Wikipedia volunteers have experience with. Over the years, a number of tools and processes have been developed to quickly spot and revert vandalism on the site. Most vandalism on Wikipedia is corrected within five minutes, as we saw today," they went on.