Istanbul Governor Vasip Sahin described the attack in Ortakoy as “cruel” and an “act of terror”.
“At 1.15 a.m. [2215GMT], a terrorist carrying a long-barreled weapon martyred the police officer waiting outside, and then martyred another citizen" to enter the club, he said.
“He then carried out this violent and cruel act by spraying bullets on innocent people who were celebrating the new year," Sahin added.
An investigation is underway and the governor’s office would provide further updates, he went on to say.
Pakistan has condemned in the "strongest possible" terms the heinous terrorist attack on the Reina nightclub.
"We are deeply anguished as terror and tragedy have struck brotherly Turkey again," Pakistan's foreign ministry said.
"We reaffirm our strong and unwavering solidarity with the Government and people of Turkey in its resolute fight against terrorism and reiterate our belief that such heinous acts aimed at undermining Turkey's peace, stability, progress and internal harmony would never succeed," the statement added.
Pakistani Prime Minister Muhammad Nawaz Sharif has also strongly condemned the attack. "Pakistani people share the grief of Turkish people and stand by them, in this most painful time. Terrorism is our common enemy and the world needs to put up a joint fight against this menace," he said.
Russian President Vladimir Putin expressed his condolences to his Turkish counterpart Recep Tayyip Erdogan, the Kremlin said in a statement.
Putin said it was hard to imagine a "more cynical" crime than the murder of innocent civilians during New Year's holiday. "But, the terrorists are absolutely alien to the concept of human morality," he added.
The Russian leader said the common duty would be in responding "decisively" to terrorist aggression. The president said Russia remained a reliable partner for Turkey in its fight against terrorism.
South Korea's foreign ministry released a statement reiterating that acts of terrorism "cannot be justified under any circumstances".
"Our government would like to extend our condolences to the Turkish government, the innocent victims and their families," it said.
Israeli President Reuven Rivlin said in a statement that "the attack against an Istanbul nightclub is an attack against the whole humanity".
"Our hearts are with the Turkish people at this difficult time. I offer my condolences to the families of the victims and well-wishes to the injured," Rivlin added.
Israeli Prime Minister Benjamin Netanyahu said that at least one Israeli woman was hurt in the attack. The fate of another Israeli woman still remains unknown, he added.
Greece issued a condemnation message, expressing their full solidarity with the Turkish people and sent out sincere condolences to the families of the victims.
Qatar also issued condolences to the victims' families and the Turkish government and people.
The Gulf country stressed "the need to strengthening coordination among world countries, and to unite the international community's efforts in order to ensure the eradication of terrorism and its causes".
Kazakhstan also strongly condemned the terrorist attack. "The world has seen yet another manifestation of terrorism in this heinous crime whose organizers ... should be found and brought to justice," the country's foreign ministry said in a statement.
Georgian President Giorgi Margvelashvili tweeted: "We are in solidarity with President Recep Tayyip Erdogan and [the] Turkish people."
Boris Johnson, U.K. Foreign Secretary wrote on Twitter: "Thoughts with Turkey after cowardly act of terrorism in #Istanbul nightclub attack. We stand shoulder to shoulder with our Turkish friends."
President of Azerbaijan, Ilham Aliyev said: "We are extremely angry because of this treacherous attack against civilians. We support the toughest, decisive and continuous fight against terrorism."
Iranian Foreign Ministry spokesman Bahram Qasemi issued a statement, saying, he "underscored that unity among all world countries is the sole way to tackle violence and terrorism across the globe".
White House spokesman Eric Schulz said U.S. President Barack Obama “expressed condolences for the innocent lives lost, directed his team to offer appropriate assistance to the Turkish authorities, as necessary and keep him updated as warranted".
Obama "reaffirmed the support of the U.S. for Turkey, our NATO ally, in our shared determination to confront and defeat all forms of terrorism," National Security Council (NSC) spokesperson Ned Price said.
U.S. State Department spokesman Mark Toner said Washington “strongly condemns the terrorist attack”.
"Sadly, this heinous attack is only the latest effort to kill and maim innocent civilians. These attacks only reinforce our strong determination to work with the government of Turkey to counter the scourge of terrorism," Toner added.
NATO Secretary General Jens Stoltenberg posted a Twitter message, lamenting a “tragic start to 2017 in Istanbul”.
"My thoughts are with those affected by the attack on people celebrating New Year and with the Turkish people," Stoltenberg said.
President of the European Parliament, Martin Schulz stated on Twitter: "It was supposed to be a night of celebration: terrorists turned it into one of violence, death, despair. My thoughts with Istanbul, Turkey."
European Commission President, Jean-Claude Juncker expressed his "heartfelt" condolences to Turkey in a statement.
"It is our common responsibility to repel terrorism. Let me assure you and through you the people of Turkey that you can count on our continued support in this endeavor," Juncker said.
EU foreign policy chief Federica Mogherini tweeted: "Our thoughts are with victims and their loved ones. We continue to work to prevent these tragedies.”
British Ambassador to Turkey, Richard Moore tweeted: "Saddened and shocked to see more pain and heartache visited on Turkey as we enter the New Year."
"I condemn utterly this murderous attack. My heartfelt condolences to families of deceased and wishes for a speedy recovery to those injured," Moore added.
French Foreign Minister Jean-Marc Ayrault voiced solidarity with Turkey in an online post: "With all heart we are with Turkey which was terribly struck in this symbolic night."
Canadian Foreign Minister Stephane Dion on Twitter wrote: "Canada offers its condolences to families of victims and wishes a speedy recovery to those injured in tonight's horrendous attack in Istanbul."
Foreign Minister of Austria, Sebastian Kurz said: "Terrible beginning of the New Year in Istanbul. My sincere condolences to the friends and families of the victims."
Belgium's Deputy Prime Minister and Foreign Affairs Minister, Didier Reynders stated: "The year began in horror in Istanbul. Our thoughts are with the victims, their relatives and the people of Turkey."
Minister for Foreign Affairs of Sweden, Margot Wallstrom stated on her Twitter account: "I strongly condemn the terror attack in Istanbul. Our thoughts are with victims, their loved ones and Turkey."
France expressed its solidarity in a statement, saying President Francois Hollande strongly denounced the terrorist act.
Italy's foreign minister, Angelino Alfano, wrote on Twitter: "Tears are not enough. We must continue to fight against terror. Fighting together to defend our freedom."
German Chancellor Angela Merkel sent her condolences to the victims of the Istanbul killings, saying "terrorists have carried out an inhumane and devious attack on people who wanted to celebrate the New Year together".
"My thoughts this morning are with the victims, their families and friends," she added.
Hungarian Prime Minister Viktor Orban issued a statement, sending his condolences to the families of the victims.
"In Hungary, we appreciate that Turkey and the Turkish people take an active role in the fight against international terrorism. This inhuman attack confirms my government's commitment to support Turkey's efforts and action against terrorism," Orban added.
Palestinian President Mahmoud Abbas condemned the attack. According to the WAFA news agency, Abbas reiterated his rejection of all kinds of terror activities and called for more regional and international action against terrorism.
National Executive Director of the Council on American-Islamic Relations, Nihad Awad, said on Twitter: "Our hearts with the families and loved ones of the civilians killed and injured by terrorists in Istanbul."
Tim Cook, CEO of Apple, wrote in Turkish: “We share your pain and sadness, dear Turkey. Istanbul, our hearts are with you.”
The attack comes three weeks after twin bombings in Istanbul killed at least 45 people, most of whom were police officers.
Pope Francis condemned the attack in Istanbul, saying: “Deeply saddened, I express my closeness to the Turkish people.
"I pray for the many victims and the injured and for the whole nation in mourning, and I ask the Lord to support all people of goodwill who courageously roll up their sleeves to face the plague of terrorism and the bloody stain that envelops the world with a shadow of fear and bewilderment."
China's Foreign Minister has sent a condolence letter to his Turkish counterpart, Mevlut Cavusoglu.
Moldovan President Igor Dodon also sent a condolence letter to President Erdogan.
Somalia's Information, Culture and Tourism Minister Mohammed Abdi-Hassan sent a condolence letter to Turkey's embassy in Mogadishu, condemning the terror attack.
President of Tatarstan, Rustam Minnikhanov, also sent a condolence letter to the Turkish leadership.
Australian Foreign Minister Julia Bishop condemned the terror attack via her social media account, saying: "Australian Government condemns deplorable terrorist attack #Istanbul. Condolences to families of those killed, injured."
Ukrainian, Kosovan, Albanian, Danish, Swiss officials have all condemned the attack.
OSCE Chairperson-in-Office Sebastian Kurz and Secretary General Lamberto Zannier on Sunday strongly condemned the horrible attack on the Reina nightclub.
“Once more, Turkey has been shaken by the scourge of terrorist violence. I condemn in the strongest terms this brutal and perfidious attack against innocent people celebrating the beginning of the New Year.
"Together, we stand with our Turkish friends in these hours of mourning”, Kurz said.