The polls have opened in the Netherlands in a referendum on a far-reaching free trade deal meant to foster closer ties between Ukraine and the European Union.
Dutch opponents of the EU-Ukraine association agreement argue its ultimate goal is bringing Kiev into the EU.
Supporters say it is not a membership stepping stone and will boost trade and help battle corruption and improve human rights in the former Soviet republic on Europe's restive eastern edge.
Prime Minister Mark Rutte said after voting: "It's about solidarity with a country which wants to develop itself and I believe, in (the) longer term, I would like for Ukraine to have both a stable relationship with Europe and with Russia."
Dutch politician Geert Wilders, a fierce opponent of the EU, said he hoped the vote would give hope to other nations questioning their place in Europe.
After casting his ballot at a school on the outskirts of The Hague, Mr Wilders said the Dutch referendum could act as an incentive to British voters to reject the European Union in June.
"So it could be today that it is the start of the end of the European Union as we know it today and that would be very good," he said.
Much of the deal between the EU and Ukraine already is being provisionally implemented but the Netherlands' ratification, approved last year by both houses of Parliament, was put on ice pending the outcome of the referendum.
Exactly what will happen to the agreement if the Dutch vote against it remains unclear, but politicians all say Mr Rutte's coalition government will have to take the result seriously. The advisory referendum is declared valid only if voter turnout is over 30%.
The Netherlands is a founding member of the European Union, a trading nation that benefits from the EU's internal market, but paradoxically it also is a hotbed of Euroscepticism that rejected the bloc's proposed constitution in a 2005 referendum.
In an interview earlier this year with a Dutch newspaper, EU Commission president Jean-Claude Juncker warned that a "No" vote "would open the door to a great continental crisis".
The referendum, the first in the Netherlands since the country rejected the EU constitution, was forced by a loose coalition of Eurosceptics that managed to gather nearly 430,000 signatures in just six weeks last year.