Qubad Talabani, deputy prime minister of the KRG, called it a "historical day" as he voted at a polling station in Sulaymaniyah.

"It is the beginning of a struggle today in which we hope after a talking process with Iraq, with our neighbors, friends and rivals, to be able to reach our nation's objectives, be able to fulfill the dream that grew with us since childhood," he told reporters in comments reported by Kurdish media outlet Rudaw. "Today marks the first phase in a long-term process."

He said the referendum needn't be viewed as a threat by neighboring countries.

"We just ask our nation a question: do you want to live in an independent state and today our nation will be answering that question. It is not that we are declaring independence tomorrow," he said.

Fears from regional powers

The vote comes as Kurdish forces play an instrumental role in the fight against ISIS in Iraq and Syria. In helping to eliminate the terror group, Kurdish leaders appear to have expected the backing of the international community in pursuing nationalist aspirations.

But that support has not been forthcoming, with many world and regional powers opposing the vote and warning that it could further destabilize the Middle East.

Both Iran and Turkey have sizable Kurdish minorities and fear a vote for independence in Iraq might galvanize movements in their countries.

The United States, United Kingdom and the United Nations also warned the KRG against holding the referendum, citing fears that it could detract from the campaign against ISIS.

On Sunday, Iran closed its air space to the autonomous Kurdistan region after issuing several condemnations against the vote.

And as voters cast their ballots Monday, Turkish President Recep Tayyip Erdogan described the referendum as "illegal" and suggested Turkey could cut off oil exports from northern Iraq, depriving the KRG of a key source of revenue.

Northern Iraq "is not the living space of only one person or of one tribe, and this should be known," Erdogan said during remarks in Istanbul. "We consider the referendum, regardless of outcome, null and void. We say it is illegal."

Israel is the only country in the region that supports the vote, with Israeli Prime Minister Benjamin Netanyahu endorsing what he described as "the legitimate efforts of the Kurdish people to achieve their own state."

'This is a great day in my life'

On Monday, hundreds of voters lined up outside a polling station in Irbil, many wearing traditional Kurdish dress.

"These clothes are a language of their own. It expresses who we are and what we want," said Hiro Mahmoud who was decked out in traditional dress.

Meanwhile, ex-Peshmerga fighter and former Kurdish MP Aso Karim Mohamad told CNN the vote was a celebration.

"To see the crowds and the parties standing shoulder-to-shoulder, this is a great day in my life," he said.

According to officials, people voted at 2,000 polling stations across the region.

Abdel Kader Dizaye, a 75-year-old farmer, described it as the "biggest and happiest day of my life."

"This is a dream of a lifetime," he added.

But others were skeptical of the vote, claiming that an independent Kurdistan would turn non-Kurds into second-class citizens.

"The policies of the Kurds in Kirkuk is Saddam Hussein's policy," said Ali Mehdi of the Iraqi Turkmen Front, referring to the late Iraqi dictator who was routed from power after the US-led invasion in 2003.

"Kirkuk is very tense at the moment and the Kurds are pushing the downtrodden non-Kurdish elements to participate in the referendum," Mehdi said.

Those sentiments were echoed by Arab tribal leader Sheikh Burhan Mezher who showed CNN anonymous Facebook messages with threats to harm his children.

He said he didn't believe the threats were coming from officials but that tensions were running high and "more could be done."