Turkey's pro-Kurdish HDP opposition said on Sunday that the result of a referendum on granting sweeping powers to the president would remain unclear until its appeal to the High Electoral Board over vote irregularities was finalised.

"The referendum result is a clear sign that a societal agreement could not be reached. Our co-chairs being jailed, the referendum being held under a state of emergency, and other oppressive measures cast a shadow and legitimacy problem over the vote," HDP spokesman Osman Baydemir told reporters.

According to the state-run Anadolu agency, "Yes" votes were ahead at 51.3 percent with 99 percent of ballot boxes opened.