The pan-European party says it is "deeply concerned" about the extent of the reprisals that it says are being carried out in Turkey.
The PES points out that it "clearly" condemned the coup from the outset but that "European Socialists and Democrats are now worried about the current situation in the country."
The PES says it has "deep concerns" about the possibility that a "massive violation" of human rights may be taking place in Turkey after last month's botched coup.
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In a statement, issued on Wednesday, it said, "The huge number of people who have been detained since the attempted coup should be treated according to all human rights conventions."
It goes on, "The PES will be monitoring all reports in this respect."
However, the head of a top European rights watchdog has backed a "cleaning up" of Turkish institutions after the failed coup, which was blamed on supporters of US-based preacher Fethullah Gülen.
Despite growing concern over the post-coup crackdown, Council of Europe chief Thorbjørn Jagland said there had been insufficient understanding in Europe about the challenges faced by Turkey.
Almost 26,000 suspects have now been rounded up after the coup, which Ankara blames on followers of Gülen, who built up a presence in key institutions including the military. Gülen denies the accusations.
Jagland's comments accepting the need for a crackdown contrasts with the tone of several EU officials who while condemning the coup have expressed alarm over the scope of the arrests.
"I recognise that of course there is a need for taking on those who were behind this coup and also on this secret network," he said.
"I would like to say there has been too little understanding from Europe over what challenges this has caused to the democratic and state institutions of Turkey," said Jagland, referring to Gülen's group.
Separately, PES President Sergei Stanishev, a former Prime Minister in Bulgaria, was more critical, saying: "Human rights and separation of powers are as important to democracy as the free elections and respecting will of the people.
"There are thousands of ordinary public servants, teachers and academics who have been detained, fired from work or banned from traveling abroad. We hope that each of those cases will be processed separately with respect to the rule of law and civil rights."
Stanishev stressed that it was not just democracy that should be upheld during elections.
He said: "We strongly supported the Turkish institutions against the coup because we believe in the free will of Turkish people, but democracy is also about the separation of powers, the rule of law and fundamental freedoms such as freedom of expression.
"The further crackdown on the internet confirms our worst suspicions about the authoritarian direction that Turkey is moving in. This will only lead to Turkey and the EU moving further apart."
Stanishev also commented on the statement Turkish President Racep Tayyip Erdogan made earlier this week criticising EU foreign affairs chief Federica Mogherini for not visiting Turkey after the attempted coup.
Speaking with Italian TV Rai News 24, Erdoğan said, "Mogherini should first have come to Turkey. Now I ask: 'What would be the reaction if the Italian parliament was bombed'?"
But Stanishev hit back, saying, "Mogherini was one of the first EU officials to react against the coup in support of the democratically elected government in Turkey.
"At the same time her wise words on the rule of law have evidently not been listened in Ankara, otherwise we would not be facing such a dramatic situation in Turkey.
"President Erdogan should understand that his threats will never work against European leaders who are strong supporters of democratic principles and human rights", Stanishev said.
On Wednesday, Austria's Chancellor Christian Kern called on the EU to end membership talks with Turkey. Kern said Turkey "wasn't even a serious potential accession candidate."