Many Twitter users in Europe woke up to some unexpected Tweets: many Twitter accounts suddenly posted messages which attacked the Dutch and German governments. Unknown attackers were able to gain access to the millions of accounts through an app which provides statistics.
As of this writing, some unknown attackers were able to compromise an external service which is used for collecting statistical data. Many high-profile accounts were affected, such as UNICEF as well as Amnesty International. In addition to the profiles of sports clubs and human rights organizations, numerous other profiles were also compromised. Some celebrities suddenly appeared to post those polemic messages. Several thousand accounts were affected worldwide. The attackers replaced the header image with the Turkish flag. This process is also known as "defacement". They then proceeded to berate the Dutch as well as the German government and called them "Nazis".
An external app by the name of TwitterCounter was hacked. The provider has acknowledged the hack. An investigation is already underway. More details on the exact timeline of the attack are unknown at this time.