The narrow margin endorsing a referendum to change the country's constitution sparked rancorous debate on Twitter and Facebook, as the country's main opposition party called on Monday for the vote result to be annulled.
Opposition supporters took to the streets of Istanbul to bang pots and pans, a traditional form of protest, in a series of noisy demonstrations.
Turkish President Recep Tayyip Erdogan called victory on Sunday after 51.4 percent of Turkish voters backed switching to a presidential system and vesting the president with powers to appoint top judges, declare a state of emergency, dismiss the parliament, remain party leader while in power and serve extra terms.
Mr. Erdogan's victory was far tighter than expected, emerging only after several nail-biting hours late Sunday which saw the "No" result dramatically catch up in the later count.
The vote allows Erdogan, who has ruled Turkey first as prime minister and now as president since 2003, to fulfill his long-held ambition for a presidency with executive powers.
"On April 17, we have woken up to a new Turkey", wrote pro-government Hurriyet columnist Abdulkadir Selvi.
Most of the changes, including 18 constitutional amendments, will take effect after the next presidential election in November 2019, but as soon as the final election results are published, Erdogan can rejoin the right-wing Justice and Development Party he founded. "Because we do not have the authority to forgive the murders of our martyrs", Erdogan said at a rally Monday.
The outcome has laid bare deep political divisions in Turkey and could have wider resonance in everything from Turkey's decades-old bid for membership in the European Union and Turkish interactions within North Atlantic Treaty Organisation to the US -led coalition fighting the Islamic State in neighboring Syria. The electoral board accepted a stunning number of ballots that lacked an official stamp.
In a separate statement, France›s Foreign Ministry called on the Turkish government to respect the European Convention on Human Rights and its ban on the death penalty.
The changes - due to be introduced before presidential and parliamentary elections in November 2019 - will turn Turkey into a presidential republic similar to the U.S. and France.
Guven had said the "ballot papers and envelopes that were claimed to be void and disputed since the evening are YSK-made, real, legitimate, non-fake ballots".
The EU urges Turkish leaders to seek a "broad national consensus" and investigate the election irregularities reported by global observers, the European Commission said on Tuesday (19 April). Monitors from the Organisation for Security and Cooperation in Europe, who also listed numerous problems, said the move undermined important election safeguards.