People are voting in a landmark referendum on independence for the Kurdistan Region of Iraq - a move which has been criticised by foreign powers.
Polls are open in the three northern provinces that make up the region, as well as disputed areas claimed by the Kurds and the government in Baghdad.
Iraq's prime minister has denounced the referendum as "unconstitutional".
Kurdish leaders say an expected "yes" vote will give them a mandate to start negotiations on secession.
Kurds are the fourth-largest ethnic group in the Middle East, but they have never obtained a permanent nation state.
In Iraq, where they make up an estimated 15% to 20% of the population of 37 million, Kurds faced decades of repression before acquiring autonomy in 1991.
Voting in Monday's referendum is open to some 5.2 million Kurds and non-Kurds aged 18 or over who are registered as resident in Kurdish-controlled areas of northern Iraq. Polling stations are expected to close at 18:00 (15:00 GMT).
"We have been waiting 100 years for this day," one man queuing to vote at a school in the Kurdistan Region's capital, Irbil, told Reuters news agency.
"We want to have a state, with God's help. Today is a celebration for all Kurds. God willing, we will say yes, yes to dear Kurdistan."
Not all Kurds are expected to vote "yes", though.
The Change Movement (Gorran) and Kurdistan Islamic Group parties raised objections to the timing and organisation of the referendum, while businessman Shaswar Abdulwahid Qadir launched a "No4Now" campaign because of the economic and political risks of secession.
And in the disputed city of Kirkuk, the local ethnic Arab and Turkmen communities have called for a boycott.
Iraqi Prime Minister Haider al-Abadi warned on Sunday that the referendum "threatens Iraq, peaceful co-existence among Iraqis, and is a danger to the region".
"We will take measures to safeguard the nation's unity and protect all Iraqis."
The central government has demanded that all international airports and border crossings be returned to its control, and asked all countries to "deal only with it on matters of oil and borders".
Neighbouring Turkey and Iran also vehemently object to the referendum, fearing it will stoke separatist feeling among their own Kurdish minorities.
On Monday, Tehran called the vote "illegal and illegitimate" and said it had closed its borders with the Kurdistan Region.
Ankara said it would consider the result of the referendum "null and void" and intended to form closer ties with Iraq's central government.
The UN Security Council warned on Thursday that the vote could hamper the fight against IS in Iraq, in which Kurdish forces have played a critical role, and efforts to ensure the return of 3 million displaced Iraqis.
But Kurdistan Regional President Massoud Barzani accused the international community of having double standards.
"Asking our people to vote in a peaceful way is not a crime," he said on Sunday. "If democracy is bad for us, why isn't it bad for everyone else?"
Mr Barzani said the referendum would not draw borders, and that afterwards there could be talks with Baghdad for a year or two. But he stressed that the "failed partnership" with the "theocratic, sectarian state" of Iraq was over.