ISTANBUL -- Dozens of members of Turkey's political opposition were arrested in dawn raids on Wednesday, as a crackdown began on those questioning the legitimacy of a referendum on Sunday to expand the powers of President Recep Tayyip Erdogan.
Mr. Erdogan has claimed a narrow 51.4 percent to 48.6 percent victory in the vote, but protesters in pockets of the country have marched in the streets every night since then to demonstrate against what they assert was a rigged election.
After warnings from Mr. Erdogan, at least 38 people accused of participating in the protests were rounded up Wednesday morning or issued arrest warrants, according to lawyers and relatives of the detained.
Though tens of thousands of people have been detained for political reasons in Turkey in recent months, these are the first political arrests reported since the referendum.
"These people are mainly those who attended the protests after the referendum and raised their voice against the referendum result on social media," said Deniz Demirdogen, a lawyer for one of the detainees, Mesut Gecgel.
"The police told the detainees that they were accused of trying to agitate people against the 'yes' vote," Mr. Demirdogen said by telephone from the police station in Istanbul where his client had been taken.
Abdurrahman Atalay, a prominent political activist who filed an appeal on Tuesday against the referendum result, was also detained. His nephew, Can Atalay, said by telephone that the police had told Mr. Atalay that he was being charged for "inciting hatred among people by claiming the referendum result is dubious."
Several of the detainees are from the United June Movement, a group formed after the mass protests in June 2013 against Mr. Erdogan's government.
The arrests will add to fears that Sunday's referendum has accelerated Turkey's descent toward authoritarianism. Mr. Erdogan and his allies say their victory will help bring stability and prosperity to the country, while their critics argue that it will give the president too much power, insulate the post from judicial scrutiny and, as a result, contribute to greater instability.
Two international observer missions said the referendum campaign had been conducted in an unfair environment in which opposition voices were suppressed.
Observers also criticized the government for holding the vote during a state of emergency that was imposed after the failed coup in July against Mr. Erdogan.
Since then, roughly 45,000 people suspected of being dissidents and of plotting the coup have been arrested, more than 150 media groups and 1,500 civil society organizations have been closed and about 130,000 people have been purged from their jobs. Anti-Erdogan campaigners faced physical intimidation and restrictions on their ability to hold rallies and to appear in the news media.
An Interior Ministry official said that no one was available to comment by telephone on the arrests, asking that requests for comment be submitted by email.