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.

Binali Yildirim made the call during an address to legislators from his ruling party, as the country's main opposition party prepared to apply to the country's highest electoral board to seek an annulation of Sunday's vote which gave President Recep Tayyip Erdogan's "yes" camp a narrow win.

The Republican People's Party, or CHP, called for the vote on the referendum to be annulled citing a series of irregularities, particularly an electoral board decision to accept ballots that didn't bear official stamps, as required by Turkish law.

Monitors from the Organization for Security and Cooperation in Europe, who also listed numerous irregularities, said the move undermined important election safeguards, drawing a harsh rebuke from Erdogan.

"Efforts to cast a shadow on the result of the vote by spreading rumors of fraud are futile and in vain," Yildirim said. "The will of the people was freely reflected into the ballot boxes and this business is over. Everyone and all sections — and the main opposition party in particular— must show respect. It is wrong to speak after the people have spoken."

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.

The referendum approves 18 constitutional amendments that allow the president to appoint ministers, senior government officials and to hold sway over who sits in Turkey's highest judicial body, as well as to issue decrees and declare states of emergency.

The new system takes effect at the next election, currently slated for November 2019. Other changes are to be implemented sooner, including scrapping a requirement that the president not be a member of any political party. This would allow Erdogan to rejoin the governing AK Party he co-founded, or to lead it.

On Tuesday, Yildirim said Erdogan would be invited to join the party as soon as the official results are declared.

"We will invite our founding chairman to our party and we will feel a huge elation to see him among us," he said.

Meanwhile, OSCE monitors were seen entering the Supreme Electoral Board headquarters. Tana de Zulueta, head of the observer mission, told reporters that the group had paid a courtesy call and held a "cordial" meeting with board members.

Asked to comment about Erdogan's rebuke, de Zulueta said: "I don't have an opinion, we are invited by the Turkish authorities to observe. We share our report and we completed our mandate." 

In Istanbul, hundreds of "no" supporters demonstrated in the streets on Monday, chanting "thief, murderer, Erdogan" and banging pots and pans.

President Donald Trump, meanwhile, ignored the concerns about voting irregularities and congratulated Erdogan on his referendum victory. The two leaders also discussed Turkey's support of the U.S. response to a Syrian chemical weapons attack and efforts to counter the Islamic State group, according to the White House statement on their phone call Monday.