Turkey’s President Recep Tayyip Erdogan claimed victory on Sunday in a referendum on a proposal to massively expand his power, while dismissing the objections of opposition parties who challenged the outcome of the vote.

Erdogan’s victory sets in motion a transformation of Turkish politics, replacing the current parliamentary system with one dominated by a powerful presidency. According to preliminary results, a small majority of Turkish voters approved the set of 18 constitutional amendments that limits parliament’s oversight of the executive, eliminates the office of the Prime Minister, and expands presidential power over judicial appointments. Erdogan and his supporters say the constitutional changes are needed to ensure stability, while opponents denounced the amendments as a step toward an era of autocracy.

The narrow, disputed outcome of the vote also sets the stage for a bitter struggle over the validity of the referendum results. According to Turkey’s state news agency, the yes vote won by a margin of 51.2% to 48.8%. However, two opposition parties said they would challenge the result, citing violations in the vote-counting procedure. The campaign also took place in the wake of a vast political crackdown in Turkey following a failed military coup last July. The questions about the referendum’s results now promise to sow even more division in a country already deeply polarized over the figure of Erdogan and the merits of his proposed presidential system.

Addressing his supporters on Sunday night, Erdogan brushed aside questions of legitimacy, claiming a definitive victory in the referendum. “The discussion is over. ‘Yes’ has won.”

Throughout the referendum campaign, Erdogan has argued the new system of government would introduce political stability and security. It certainly promises to make Erdogan the undisputed leader of Turkey for years to come, inviting comparisons to Vladimir Putin of Russia and other populist autocrats.

“Who’s going to stop Erdogan? There never was anyone to stop Erdogan, but now, even the formal possibility of there being something is erased from the law,” says Selim Sazak, a fellow at the Delma Institute, an Abu Dhabi–based think tank.

The dispute over the outcome of the referendum centers on a last-minute decision by the state election board to count ballots that did not receive an official authenticating stamp. The country’s largest opposition party says that as many as 1.5 million ballots did not receive such a stamp, a number that would more than account for the margin of victory in the margin of victory of 1.3 million reported by the state news agency. “At least half the country said no to constitutional change. This shouldn’t be carried against the public’s will,” said Kemal Kilicdaroglu, leader of the centrist Republican People’s Party, in a televised address on Sunday night. Angry demonstrations erupted late Sunday night in neighborhoods of Istanbul where the opposition is heavily represented.

“This is a very close call, so I don’t think people are going to let it go necessarily. It will probably be talked about for some time,” says Selim Koru, an analyst at the Economic Policy Research Foundation of Turkey. He adds, “The President is obviously going to continue and try to enact a transition to make everything irreversible as quickly as he can.”

The entire referendum campaign took place amid political crackdown in the aftermath of a deadly military coup last July that failed to dislodge Erdogan and killed more than 200 people. After surviving the coup attempt, Erdogan moved to consolidate power, with authorities jailing thousands and dismissing tens of thousands of civil servants, soldiers, police officers, teachers, justice officials and others from their jobs. In a parallel set of court cases, hundreds of members of one major opposition party — the Peoples’ Democratic Party — have been imprisoned on terrorism charges, among them Members of Parliament. The government accuses the party of ties to outlawed Kurdish militants who are engaged in a long-running war with the Turkish state.

The results of the national vote also suggest some weaknesses in the President’s base of support. In Istanbul, Turkey’s largest city, where Erdogan came of age and rose to stardom as the elected mayor in the 1990s, the no votes edged out the yes votes. The “no” campaign also won the capital, Ankara, as well as Izmir, a major coastal city. A significant number of supporter’s of Erdogan’s own party, the Justice and Development Party (AKP), voted against the constitutional changes, signaling distrust with a the expansion of the power of a President who already has unrivaled control.

In Istanbul’s Kasimpasa neighborhood, where Erdogan lived as a teenager and a young man, some of the President’s supporters said they voted no.

“A presidential system doesn’t sound right to me,” said Nazli Kaya, 32, standing outside a polling station in a school in Kasimpasa. “I believe in diversity. I don’t want a one-man system,” she says.

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