Turkish Police raided the office of a left-wing opposition news website on Thursday, breaking into the office and searching the newsroom, the group said.
The news came amid reports of detentions for the supporters of an opposition presidential candidate.
Sendika.org said that there were no journalists present at the time of the raid.
The website said the raid was part of a legal investigation into its editor, Ali Ergin Demirhan.
Mr Demirhan was briefly detained in April 2017 on charges of not recognising the result of a referendum to switch to an executive presidency system.
He was separately detained in May this year for a news story titled “We can stop the dictatorship,’’ but later released.
A big majority of mainstream media outlets in Turkey are considered to be close to Islamic Conservative President Recep Tayyip Erdogan.
Critical journalists often switch to online platforms; however, many such websites are blocked in Turkey.
Meanwhile, local media reported that five people were officially arrested on Wednesday in Izmir Province for “insulting a statesman,” bringing the total number to 12.
Seven people were earlier arrested after they allegedly “cursed Erdogan” following a rally by the main opposition Republican People’s Party (CHP) presidential candidate, Muharrem Ince, on June 21.
Mr Erdogan was re-elected on Sunday, while his ruling Justice and Development Party (AKP) fell short of gaining an absolute majority in parliament.
Mr Erdogan could still rule in alliance with the Nationalist Movement Party (MHP).
Several media outlets were shuttered and dozens of journalists jailed after the 2016 attempted coup in Turkey.