Belgrade - Driven back by police batons and snarling dogs, or beaten and robbed by the smugglers they relied on, migrants caught in Serbia have regularly been victims of violence as they struggle to reach the heart of Europe.
About 8,000 migrants have been trapped in the country since the European Union closed its borders, hoping to block the so-called Balkans route taken by hundreds of thousands of people fleeing the Middle East, Asia and Africa.
But migrants continue to cross the region in smaller numbers - a few hundred a day - often with the help of traffickers.
"I could not imagine that European police could be so violent," Najim Khan, a 21-year-old mason from Pakistan, said in a Belgrade park.
The claims from migrants as well as aid groups are dismissed by the authorities: Croatia says there is "no proof" of abuses, Hungary says it acts "with respect to human dignity", and Bulgaria says it has looked into every claim "but they were never confirmed".
Khan, who arrived from Bulgaria, says that one evening, the police burst into the squat where he was staying in Sofia.
"They beat us, took us to a police station and then to a closed centre. They beat us again during transfers," he told AFP.
Once in Serbia, he tried to reach the European Union despite the increased patrols at Hungary's border, but his group was quickly spotted by the Hungarian police.
"They made us lie on our stomachs, in a line. They ran on our backs, laughing. They were throwing beers in our faces," Khan said.
"They took our cellphones and broke them. They did not take our money."
In early March, medical aid group Medecins sans Frontieres (MSF) denounced the violence against migrants, calling it a "ritual of brutality... designed to stop people from trying to cross again".
'Maybe they wanted money'
"The militarisation of the EU borders has led to a staggering increase in violence, especially along the Balkans," said Andrea Contenta of MSF, which has set up a clinic in Belgrade.
"More than half of our patients have experienced violent events during their journey."
Rados Djurovic, of the Asylum Protection Centre in Serbia, said migrants "complain mostly about violence suffered in Hungary, where they were bitten by dogs, hit brutally, causing broken bones".
Many also complained about abuse in Croatia, but the situation was better in Serbia, where the police have been given clear instructions, according to an aid worker who declined to be named.
Contenta added that although smugglers were responsible for some of the assaults, "the vast majority of our patients reported alleged violence perpetrated by state authorities, mainly by EU member states such as Hungary, Bulgaria and Croatia".
Attal Shafihullah, a 16-year-old Afghan, said he had experienced both.
One night Shafihullah and three comrades were intercepted by the Bulgarian police as they tried to leave Serbia.
"Sometimes they let you go," he said. "Other times not."
This time, the officers beat them, he said.
"Maybe they wanted money," said Shafihullah, whose face bears the scars of burns suffered when his home went up in flames in Afghanistan.
But he is certain that financial motivations were behind the blows of smugglers he met a few weeks later, as they told the migrants to have money sent to them from back home.
"They wanted to make an example, to show that it is a serious business," Shafihullah said.
In a Belgrade reception centre, 14-year-old Qayum Mohammadi remembered vomiting after being sprayed with tear gas when the bus carrying him and other migrants crashed into a wall while trying to outrun a Bulgarian patrol.
Some weeks later, in Hungary, officers made him lie on the ground before sending him back to Serbia, added the teenager with a budding moustache.
"They told me to put my hands on the ground, and then walked on them... they hit my thighs with a baton" before sending him back to Serbia, he said.
Rights groups say the EU border closures have only made the Balkans route more dangerous, now that such attempts are illegal.
Medecins sans Frontieres has registered more than 70 migrant deaths between Greece and Hungary since last year.
The causes of death include hypothermia, drowning, traffic accidents - and suicides.
By Nicolas Gaudichet