Istanbul police on Wednesday detained 16 leftist activists involved in demonstrations against the 'Yes' victory in the referendum on expanding President Recep Tayyip Erdogan's powers, a laywer said.
There have been daily street protests in anti-Erdogan neighbourhoods in Istanbul since Sunday's referendum, which the opposition claims was marred by blatant vote rigging.
The Freedom and Solidarity Party (ODP), a left-wing movement not represented in parliament, said police had detained its Istanbul chairman Mesut Gecgel on accusations of "agitating the public" by claiming the 'Yes' vote was illegitimate.
Gecgel had himself written on Twitter that he was being detained for protesting against the 'Yes' victory.
Gecgel's lawyer Deniz Demirdogen told AFP that anti-terror police raided houses of the suspects before dawn, saying 16 people were detained so far but arrest warrants were issued for a total of 38 people.
Describing the accusations as "strange", he added: "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."
Demirdogen said those detained were being questioned at police headquarters in Istanbul and were awaiting a decision by prosecutors on the case.
There was no immediate confirmation of the detentions by the police.
Speaking in Ankara earlier, Prime Minister Binali Yildirim said the issue of the election result was closed and warned against street protests.
"Turkey is a state of law... and there can be no talk of anarchy, activities in the street," he said.
"I call on people not to give in to provocations or get caught up in incitement," he added.
The 'Yes' camp won Sunday's poll with just 51.41 percent of the vote.
But the main opposition Republican People's Party (CHP) has formally requested that the Supreme Election Board (YSK) cancel the result over to alleged violations which the opposition says swung the result.
The opposition is particularly incensed by a last-minute move by the YSK to accept ballot documents in envelopes without an official stamp.
The YSK was considering the petition and was expected to make an announcement later in the day.
CHP spokeswoman Selin Sayek Boke vowed that the party would use all means to challenge the result and could even consider withdrawing from parliament.
"We will use all the legal paths, and all legitimate democratic rights for the millions (of voters), no one should doubt this," she told reporters in Ankara.
"This can include withdrawing from parliament or continuing to work in parliament," she added, calling for the referendum to be re-run.