The government has vehemently denied the claims, saying all those held over the July 15 coup are being treated fully in line with the law
Nils Melzer, special rapporteur on torture, arrived in Turkey on Sunday and will speak with alleged victims and inspect detention facilities, the United Nations in Ankara said.
His visit, the first by a UN torture expert to Turkey since 1998, comes a month after US-based watchdog Human Rights Watch (HRW) accused Turkish police of torturing detainees.
The government has vehemently denied the claims, saying all those held over the July 15 coup are being treated fully in line with the law.
Since a rogue faction tried to oust President Recep Tayyip Erdogan from power on July 15, Turkey has arrested some 37,000 suspects, causing international alarm.
It has also imposed a state of emergency which has already been extended once.
The rapporteur will visit places of detention such as police stations, pre-trial facilities and prisons, the UN said last week, ahead of preparing a final report for the Human Rights Council in March 2018.
His predecessor, Juan Mendez, was due to visit in October but the trip was put off by the Turkish authorities -- a move which Mendez said at the time "sends the wrong message".
But the UN said the visit would be a chance to "identify and assess... challenges related to torture and ill-treatment".
"I look forward to engaging with the Turkish government on how to meet the challenges of upholding the rule of law, promoting accountability, and fulfilling the right of reparations for victims, in particular in the aftermath of the attempted coup," Melzer said on Friday.
HRW last month cited 13 cases of alleged abuse, including torture, sleep deprivation, severe beatings, sexual abuse and rape threats among people detained.
Just over a week after the coup bid, Amnesty International said it had "credible evidence" of the abuse and torture of people detained in the sweeping arrests.
Melzer started his independent role earlier this month and currently works at the Geneva Academy of International Humanitarian Law and Human Rights.
The former Red Cross and Swiss foreign affairs official will visit several Turkish provinces, the UN said without giving further details, and he will speak with victims and their families.
He will present his preliminary findings in Ankara on Friday.