‘I call on everyone to keep the peace and to act within the boundaries of the law,’ PM Mark Rutte urged.

Dutch Prime Minister Mark Rutte on Saturday condemned an attack by masked men on a shelter housing Syrian refugees, the country’s worst such incident since large numbers of migrants began arriving in Europe this year.

A group of about 20 men hurled fireworks and eggs at the temporary refuge in a sports centre in the city of Woerden, near Utrecht, late on Friday.

City mayor Victor Molkenboer said the incident had “an enormous impact” on the refugees. “It recalled traumatic experiences for them,” he told a news conference.

Police said on Saturday they had arrested and charged 11 suspects aged between 19 and 30 over the attack, in which no injuries were reported.

Rutte was due to visit the shelter, which is home to nearly 150 refugees and one of more than a hundred makeshift refuges set up in the Netherlands in recent weeks.

Hours before Friday’s attack, Rutte called for calm after residents in the town of Oranje accosted the country’s deputy justice minister because they were angry about the unannounced expansion of a refugee shelter.

“You can get mad, but violence or threatening violence is out of bounds,” the conservative premier said in his weekly news conference. “I call on everyone to keep the peace and to act within the boundaries of the law.”

The Freedom Party of anti-immigration politician Geert Wilders has soared to first place in national opinion polls since Europe’s migration crisis intensified, and it now leads Rutte’s VVD Party by a sizeable margin.

Wilders says the Netherlands should shut its borders and reject all migrants or refugees, a stance that Rutte has rejected as undesirable and illegal.

After a sharp increase in the number of asylum seekers in the Netherlands in late August, the country has received 36,000 so far in 2015 compared with 25,000 in all of 2014, according to official figures.