The hate speech trial of Dutch anti-Islam MP Geert Wilders opened in his absence Monday, in a case that will test the boundaries of free expression ahead of parliamentary polls next year.

His lawyer Geert-Jan Knoops read out a statement by Wilders explaining why the firebrand politician has chosen not to appear in court.

"It is a political process and I have decided not to be present. It's my right as a politician to speak out if there is a problem in the Netherlands," Wilders said in the statement read by Knoops.

A three-judge bench is hearing the case against Wilders, 53, on charges of insulting a racial group and inciting racial hatred after comments he made about Moroccans living in the Netherlands.

Due to run until November 25, the trial particularly focuses on a comment made at a March 2014 local government election rally, when Wilders asked supporters whether they wanted "fewer or more Moroccans in your city and in the Netherlands?"

When the crowd shouted back "Fewer! Fewer!" a smiling Wilders answered: "We're going to organise that."

Knoops told journalists before the hearing started that Wilders decision not to attend "is not a big surprise."

"It's in line with his previous statements and we as the defence respects his decision not to appear before the court," Knoops told reporters outside the high-security courthouse at Schiphol where the trial is being held.

The venue was chosen for the security of Wilders, who has round-the-clock protection and has been dubbed the country's "most heavily guarded man".

It is the second such trial for Wilders who was acquitted on similar charges in 2011.

A handful of supporters stood outside the courthouse Monday waving banners saying: "Fewer morality knights, more patriots!" and "To Parliament, via Schiphol," in reference to Wilders' argument that his statements should be discussed in parliament and not in court.

"It's a real shame that he is being put on trial for speaking his mind," one of the protesters, who refused to be named, told AFP.

If found guilty, Wilders could face a two-year jail term or a fine of over 20,000 euros ($22,000), but experts said such a severe punishment was unlikely, as he would be regarded as a first-offender and could face a lesser fine or community service.