Mysterious armed groups are on the prowl, targeting migrants in night attacks in Calais and elsewhere in northern France, sowing fear among the displaced travelers living in squalid slums and deepening concerns the city is becoming a tinderbox of anti-migrant, anti-Muslim rage that's fueling a budding nationalist movement.

The stalkers, sometimes masked, slip through the night armed with clubs, brass knuckles, pepper spray or knives, according to accounts by migrants and groups working to provide medical and legal help.

After months of what appear to be organized attacks, police made their first arrests Thursday, taking seven men armed with iron bars and extendable batons into custody for a suspected attack on five Iraqi Kurds at Loon-Plage, a port town between Calais and nearby Dunkirk. The seven faced charges of violence in a group and forming a group to commit violence, said Dunkirk prosecutor Eric Fouard. Some of the men, aged 24-47, said they sympathized with extreme-right movements in Calais identified as xenophobic, he said.

"The ideas they peddle are that there are too many migrants in France," Fouard said by telephone, noting that one of the seven was from Brittany and another from the Paris region.

The head of a legal center set up for the refugees in the makeshift Calais camp alleged on Friday that those living there are regularly subject to police violence, as well. Marianne Humbersot told reporters she was filing 13 complaints — five for violence by militia and eight at the hands of police.

"I have a 13 year old who had his foot broken. And 10 days before being attacked by police, he had his nose broken by racists," Humbersot said.

Migrants — who have converged in northern France hoping to sneak into Britain — have also long complained about police brutality, accounts backed up by medical units that treat them. But attacks in recent months, accounts suggest, are organized and carried out by a militia-style group or groups, opening a new dimension of violence.

A growing security crackdown aimed at keeping thousands of migrants from reaching Britain is giving Calais a fortress-like look. The city bristles with tall barbed wire fences, blinks with police lights and is disfigured by open spaces cleared of brush — including at the two entrances to the camp — so police can better survey.

Among the city's population, a potentially toxic cocktail of frustration and anger is brewing, with pro- and anti-migrant groups facing off in demonstrations. On social networks, anti-migrant groups, often calling themselves "patriots," are using increasingly virulent language.

"We are playing with fire because people are becoming defensive. They are organizing themselves," said the Doctors of the World coordinator for northern France, Amin Trouve-Baghdouche.

About 150 people defied a ban on a Feb. 6 demonstration in Calais organized by the anti-Islam movement PEGIDA, which staged protests in numerous European countries that day. Police charged the Calais demonstration to end it, pulling a retired four-star general who once headed the Foreign Legion into their security net. Gen. Christian Piquemal was arrested, charged and ordered to stand trial along with four others, outraging some right and far-right politicians and his partisans. Three others, all armed, were convicted and given jail terms of up to three months.

A week earlier, about 50 migrants broke through fencing and briefly occupied a ferry, the "Spirit of Britain," at the Calais port during a demonstration by some 2,000 migrants and supporters.

There are currently about 4,200 migrants in Calais, from Syria, Iraq, Afghanistan, Eritrea and elsewhere, and up to 2,000 more in another makeshift camp near Dunkirk, with hundreds of others scattered along the coast hoping to finish their journeys in Britain.

The "jungle," as the open-air Calais migrant slum is known, embodies in one sprawling stretch of filth and mud the hardships, and horrors, of uprooted lives. And the city's tensions reflect simmering uncertainty around Europe as it absorbs 1 million Syrian refugees and other migrants who arrived last year.

The bid to keep the travelers from accessing the Calais ferry port, the Eurotunnel and trucks making the journey to Britain has frustrated migrants, leaving them to take greater risks to make the crossing. Up to 20 have died since the end of June. The local Nord Littoral newspaper said four Afghans in a small boat were saved last week. A body was found this week in the waters of the port.

Now, a new fear, being physically attacked, has surfaced.

"Today, we have organized groups ... dressed in the same way with hoods who say they are police," said Baghdouche of Doctors of the World. The men are armed with clubs, iron bars, pepper spray and knives, he said, citing numerous accounts by migrants seeking medical aid. The attacks occur in town or near the jungle camp — and now closer to Dunkirk.

"They stop them, usually late, at 11 p.m. to 3 a.m., say they are police so obviously the migrants stop. They ask them to undress ... they start to hit them until they fall down, KO (knockout)," Baghdouche said.

The aid group Doctors of the World has helped migrants in the area file at least two legal complaints about attacks.

Prosecutor Jean-Pierre Valensi said he has sent four to five cases for investigation by judicial police over several months, the latest on Wednesday when a group of migrants was attacked with clubs near the camp.

"We have rather vague descriptions (of the attackers). The migrants disappear. We need accounts. We can't find them," he said.

The victims, who are in France illegally, fear filing formal complaints, in part to avoid being trapped here or sent home. Since they are illegally in the country they have no right to eventual reparations.

Some Calaisians worry migrants are hurting the economy of Calais, a tourist destination for British that, it is widely agreed, has a growing image problem. Others worry about their children's safety, noting streets empty in the evening, or property values.

"The migrants are more and more violent. They are blocked here. It's a dead-end," said Sandrine Desert, a founder of Calasiens en Colere (Angry Calasians), a group that cruises town to film migrants in the streets and post them on Facebook. "We are the forgotten ones ... . They always talk of migrants, but we are also in distress."

Philippe Wannesson, who blogs daily about his town, said, "The public debate is becoming far more divided."

"They're not ready to fight," he said, but there is "increasingly violent" extreme-right talk.