The Freedom Party leader has dismissed the court proceedings as politically motivated.
The trial comes 18 months after he led a chant calling for fewer Moroccans in The Netherlands, branding them scum.
It's being held at a high-security courtroom next to Amsterdam's Schiphol Airport - a site usually reserved for the most serious cases of organised crime, and trials of militant Islamists.
The lawyer acting for Mr Wilders, Geert-Jan Knoops says his client believes the court isn't the right place to contest his views.
"One of his main arguments is that the charges which are laid against him should be actually (tested), to put it that way, within the parliament and not before the court of law. That is his point of view."
But prosecutors say there's nothing political about the trial, stressing they had to act.
Spokeswoman Alexandra Oswald says freedom of speech has its limits.
"Politicians have immunity for what they say in parliament. He said this outside parliament. This is a collision between constitutional rights. Freedom of speech is an important thing but discrimination is not allowed. We feel that with what he said, he's made racial comments about a group of people."
But Mr Wilders insists this is entirely political, and an attempt to strip him of his right to free speech.
He denies the charges, issuing a staement saying he's only relaying what millions of Dutch people think.
"This is why I have said nothing wrong. Millions of Dutchmen, 43 per cent of the population, want fewer Moroccans. Not because they hate all Moroccans, or want all Moroccans to leave the country, but because they are fed up with the nuisance and terror of that many Moroccans. Just like I am. If speaking about this is punishable, then the Netherlands is no longer a free country, but a dictatorship. This trial is a political trial. I refuse to cooperate with it. Political points of view should be discussed in the parliament, not in court."
Although Geert Wilders has never governed, his tough line on immigration and Islam has set the tone of political debate in the Netherlands for a decade.
He claims the roughly 400,000 predominantly Muslim Dutch Moroccans make up a disproportionate share of welfare recipients and criminals.
The case currently before the court centres around this incident during campaigning for local government elections in 2014.
"So I ask you all, do you want in this city and in the Netherlands less or more Moroccans?"
Crowd: "Less, less, less!"
"Then we will take care of it."
Geert Wilders could face up to two years in jail and a fine of more than AU$8,000 if convicted of discrimination and inciting hatred.
A verdict is due in December, around three months before elections, when Mr Wilders is hoping to unseat Prime Minister Mark Rutte's conservative VVD party, which rules in a fragile coalition with Labour.
A new opinion poll shows Mr Wilders trailing Mr Rutte by a margin of two seats in the 150-seat legislature.
He's poised to more than double his number of seats in the lower house.
Geert Wilders has previously been acquitted of inciting racial hatred in 2011, after he called for the Koran to be banned and for "criminal" Moroccans to be deported.
