Supporters of Turkish President Erdogan have posted messages on several high-profile Twitter accounts. The hackees include Borussia Dortmund, ex-tennis star Boris Becker, broadcaster ProSieben and Amnesty International.
The hackers posted early on Wednesday morning using the hashtags nazialmanya (Nazi Germany) and Nazihollanda (Nazi Netherlands), a swastika symbol and the sentence "See you on April 16."
This is date of a referendum that Turkish President Recep Tayyip Erdogan hopes will give him enhanced constitutional powers.
"We are aware of an issue affecting a number of account holders this morning," a Twitter spokesperson said.
Hackers targeted several accounts with a large amount of followers. Among the victims were Amnesty International, the football club Borussia Dortmund and tennis legend Boris Becker.
Most of the posts have since been taken down.
"Sorry, we have probably been hacked this morning - just like @BVB and other large accounts."
Borussia Dortmund says thanks:
Some 3 million Turks live in Germany, a large number of whom are eligible to vote in the referendum. Turkish officials have been seeking to campaign in both Germany and Holland in recent weeks and have been largely blocked from doing so.
This in turn has raised tensions as the Netherlands goes to the polls and also adds grist to Erdogan's mill that the EU is seeking to undermine his rule in Turkey.
Chancellor Angela Merkel's chief of staff, Peter Altmaier, warned on Wednesday that the German government reserves the right to impose entry bans on Turkish officials hoping to campaign in Germany, though he said the measure would be a "last resort."
This follows days of escalating tensions between Turkey and two EU nations, Germany and the Netherlands, over Turkish politicians' hopes to campaign there ahead of their country's referendum.
Erdogan has accused Germany of "Nazi practices" and recently labeled the Netherlands as "Nazi remnants" after it prevented two Turkish ministers from holding campaign rallies.
jbh/rt (dpa, AP)