The sites were inaccessible for many users in Turkey, with the Hurriyet newspaper saying that service providers got an order from prosecutors to block Twitter, Facebook and YouTube.
Recommended: Why More People are Sharing less on Facebook
YouTube.com ran the text of a court ruling on its site saying an "administration measure" had been implemented by the country's telecommunications authority (TIB).
Hurriyet said the order blocked a total of 166 websites, including the three Internet giants, which published the images.
Bulent Kent, the secretery general of Internet Service Providers Union (ESB) told Hurriyet that a prosecutor's order was issued to the union, but that the process was ongoing.
Recommended: Top Brands Choosing Instagram over Facebook
He said that all service providers were expected to implement the ban immediately. Prosecutor Mehmet Selim Kiraz and his two captors linked to Marxist Revolutionary People's Liberation Party-Front (DHKP-C) were killed after a shootout at a courthouse in Istanbul on Tuesday.
The DHKP-C had published pictures showing one of the militants -- his face concealed by a scarf with the group's red and yellow insignia -- holding a gun to the hostage's head at his offices in Istanbul.
The images were circulated on social media while the siege was going on and were published by several Turkish newspapers as well as news websites. A day after the standoff, Turkish prosecutors launched a probe into four newspapers for disseminating "terrorist propaganda" after they published the controversial images.
President Recep Tayyip Erdogan and the ruling Justice and Development Party (AKP) have been repeatedly criticised for using court orders to block critical websites, topping a Google table for content removal requests requests.
Last year the search engine also accused Turkey of intercepting its Internet domain, redirecting users to other sites. The government blocked Twitter and YouTube in March 2014 after they were used to spread a torrent of audio recordings implicating Erdogan -- then premier -- and his inner circle in an alleged corruption scandal.
Turkey's parliament last month approved legislation to tighten the government's control over the Internet by allowing it to block websites without prior judicial authorisation, sparking outrage both at home and abroad.