Hundreds of people demonstrated in two separate neighbourhoods of Istanbul overnight, protesting against a narrow win for President Recep Tayip Erdogan in a referendum granting him greater powers.
Holding banners reading, "No, We will win" and chanting "Thief, murderer, Erdogan," on group of demonstrators marched through the neighbourhoods of Besiktas on the European side.
Others protested in the streets of Kadikoy on the Asian side of the city.
Opposition parties have called for the referendum to be annulled, complaining of a series of irregularities in the vote.
An electoral board decision to allow as valid more than a million ballots cast without the official stamp has caused particular outrage.
An international electoral monitoring mission from the Organisation for Security and Cooperation in Europe also reported a series of irregularities, and said the electoral board decision undermined safeguards against fraud.