Turkey's main opposition party says it will demand a partial recount in the constitutional referendum citing irregularities in the voting process - allegations the head of the electoral commission rejected on Monday.

Unofficial results showed 51.4 percent of Turks voted "Yes" to approve changes to their constitution and grant the country's presidential office extensive executive powers. Voter turnout was about 85 percent.

Kemal Kilicdaroglu - leader of the Republican People's Party (CHP), which campaigned for "No" - said he respects the electorate's will, but criticised a decision by Turkey's High Electoral Board (YSK) taken on the day to count "unsealed" paper ballots - ones not given an official stamp - which he said overshadowed the referendum's results.

"The rule of the match cannot be changed while the match is being played, which is an universal rule," Kilicdaroglu said late on Sunday, claiming the electoral board illegitimately changed the rules after the end of voting.

"I sadly want to state that [YSK] made this referendum questionable," he added.

"In order to support our democracy, we did everything that we could in unfair conditions. When I say that we did everything we could, I mean everything within the law. Those who campaigned for a 'Yes' vote, breached the law."

Turkey: Recep Tayyip Erdogan hails referendum victory

The head of Turkey's electoral board, however, defended the vote-counting process and said the referendum's results were valid.

Sadi Guven told media at the board’s headquarters in Ankara that the position on the paper ballots was a "unanimous decision made before voting results transferred to the system".

"Before counting, ballots, envelopes, and number of voters are crosschecked,” Guven said on Monday.

He said unsealed vote envelopes were also used in past elections. Guven added final results will be released within 12 days.

Under the changes, most of which will only come into effect after the next elections in 2019, Turkey's governance will be transformed from a parliamentary system to an executive presidency.

Turkey's President Recep Tayyip Erdogan has said the changes are necessary to bring stability to Turkish politics and prevent a return to fragile coalition governments.

Before the vote, critics had said creating a powerful presidency was an attempt by Erdogan to establish "one-man rule".

READ MORE: Essential reading on Turkey's crucial referendum

Speaking to Al Jazeera, Bulent Aliriza, director of the Turkey Project at the Center for Strategic and International Studies, said the outcome of the referendum confirmed Erdogan's dominance in the country's politics, but also exposed divisions within Turkish society.

"What is disconcerting for him [Erdogan] is not just the opposition, which is crying foul, but the fact that almost 50 percent of the people voted against this massive change and may not be willing to accept the result easily," Aliriza said.

Vote monitors from the Organization for Security and Cooperation in Europe (OSCE) and the Council of Europe criticised how information was disseminated prior to the referendum.

Tana de Zulueta, head of the OSCE delegation, said the "Yes" campaign dominated media coverage.

"Restrictions on the media, the arrests of journalists, and the closure of media outlets, reduced voters' access to a plurality of views," Zulueta said.

Erdogan sharply rejected international election observers' criticism of the referendum.

"This country has carried out the most democratic election - something not one country in the West has ever experienced," Erdogan said, speaking before jubilant supporters in front of the presidential palace in Ankara on Monday.

"Know your limits," he warned the observers.