It also decried the electoral board's decision to accept unstamped ballots.
"Any move to reintroduce the death penalty would be another disastrous step away from human rights norms for Turkey", Williamson said.
"Calling people to the street is wrong and is outside the line of legitimacy", Yildirim said, adding, "we expect the main opposition party's leader to act more responsibly".
ANKARA, Turkey Turkey's prime minister on Tuesday called on the opposition to respect the result of a referendum that will give sweeping new powers to the office of the president, but the main opposition party formally requested the vote be annulled.
His congratulations stands in stark contrast to the more cautious tone adopted by European leaders and a statement issued by the US State Department, which acknowledged the results but warned against further repression by the Turkish government of the political opposition.
Yildirim said the "the path to seek rights" should be limited to legal objections and urged the opposition to accept the vote's outcome.
Worldwide election monitors, including from the Organization for Security and Cooperation in Europe, noted a series of irregularities, and said the decision to accept as valid ballots without official stamps undermined safeguards against fraud and was contrary to Turkish law.
"I don't think Erdogan will change the robust and muscular approach to politics that he has adopted so far", said Fadi Hakura of the British thinktank Chatham House.
Thousands have protested in Istanbul and Ankara since Sunday's referendum, which has set into motion the transformation of Turkey's system of government from a parliamentary into a presidential one.
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The Turkish government on April 17 made a decision to extend a state of emergency for another three months upon the recommendation of the National Security Board (MGK), a day after a key referendum overhauling Turkey's governance system was narrowly approved.
The new system takes full effect at the next election, now slated for November 2019.
Under the outgoing constitution, the president had been required to remain impartial and renounce party political ties. This would allow Erdogan to rejoin the governing AKP he co-founded, or to lead it.
Turkey's state-run news agency says 19 people were detained in Istanbul for allegedly using the results of a constitutional referendum as an "excuse" to organize "unauthorized demonstrations".
"We will invite our founding chairman to our party and we will feel a huge elation to see him among us", he said.
In light of potential unrest, the National Security Council moved on Monday to extend a nine-month state of emergency first put in place in response to July's failed coup attempt.
Asked to comment on Erdogan's rebuke, de Zulueta said: "I don't have an opinion". Hundreds of demonstrators marched in a central neighbourhood, clanking pots and pans and chant. "Reintroducing the death penalty will move Turkey farther away from the European Union and its values".
Turkey's main opposition began a battle on Tuesday to annul a referendum handing President Tayyip Erdogan sweeping new powers, while the bar association and an global monitor said an illegal move by electoral authorities may have swung the vote.
The White House readout of Mr Trump's call noted the pressing issues on which the U.S. has tried to work with Turkey, namely fighting the so-called Islamic State group and quelling Syria's civil war.