The Freedom and Solidarity Party (ODP) said police detained its Istanbul chairman, Mesut Gecgel, on Wednesday on the accusation of "agitating the public" by claiming that the win for the 'Yes' campaign was illegitimate.
The head of the left-wing ODP movement, which is not represented in the Turkish parliament, had himself said in a message on Twitter that he was being detained for demonstrating against the 'Yes' victory.
Deniz Demirdogen, Gecgel's lawyer, said anti-terror police raided the houses of suspects in Istanbul in the early hours of Wednesday, adding that 16 people had been detained so far but arrest warrants had been issued for a total of 38 people.
Demirdogen said the accusation was "strange", adding, "They are accused of provoking people to question the legitimacy of the 'Yes' in the referendum."
"But there's no such crime definition in the penal code," the lawyer said.
Demirdogen said the 16 detainees were being questioned by police and were awaiting a decision by prosecutors on the case.
Police have not confirmed the detentions.
In Sunday’s referendum, the ‘Yes’ campaign won over 51 percent of the votes, while the 'No' campaign gained nearly 49 percent.
Since Erdogan declared victory in the referendum, protests have been held in the country.
Meanwhile, Turkey’s two main opposition parties officially submitted an appeal against the referendum result.
Representatives of the Republican People's Party (CHP) and the pro-Kurdish People's Democratic Party (HDP) presented their applications to the Supreme Electoral Council (YSK) in Ankara on Tuesday.
Electoral board rejects referendum appeals
Turkey’s highest electoral authority on Wednesday rejected the appeals to annul the referendum after complaints of vote-rigging.
"HDP, CHP and Vatan Party appeals regarding the April 16 referendum were discussed separately and as a result of evaluations, the appeals were rejected with 10 votes against and 1 vote in favor," the YSK said.