The three Bulgarians and one Afghan in custody over the 71 deaths are believed by police to be low-ranking members of one of the numerous human trafficking gangs that many of the tens of thousands migrants coming to Europe pay to help them.
"This is a human tragedy that requires a determined collective political response. It is a crisis of solidarity, not a crisis of numbers," Ban Ki-moon said Friday, expressing his horror at events in Austria and yet another Mediterranean shipwreck.
"I appeal to all governments involved to provide comprehensive responses, expand safe and legal channels of migration and act with humanity, compassion and in accordance with their international obligations."
The truck, found Thursday on an Austrian motorway in the baking sun near
the Hungary border, contained the tightly packed, decomposing bodies of 59 men, eight women and four children including a little girl. They were thought to be Syrians.
The grisly discovery highlighted the dangers faced by people fleeing conflict and hardship in the Middle East and Africa even once they reach Europe, with many putting their fate in the hands of profit-hungry people smugglers.
It came as yet another boat crammed with desperate migrants sank in the Mediterranean off Libya. Seventy-six bodies were recovered but the United Nations said up to 200 people on two vessels were feared drowned.
The United Nations estimates 300,000 people have fled conflict and hardship in the Middle East and Africa for a better life in Europe this year, and 2,500 have died in the attempt, mostly in the Mediterranean.
"My little sister, someone climbed on her back and pushed her down. When I saw her for the last time, she was underwater with him on top of her," Pakistani Shefaz Hamza, 17, a survivor of the latest tragedy, told AFP in Libya.
"My mother and I spent nine hours in the water, holding on to a bit of wood. I kept telling her everything would be okay. But a quarter of an hour before the rescue team arrived, she passed away," he said.
A court in the Hungarian town of Kecskemet, some 100 kilometres (60 miles) north of the Serbian border that some 140,000 have crossed this year, was due to rule on whether the four arrested men can remain in custody for further investigation.
They include the owner of the vehicle and two drivers, and were likely "low-ranking members... of a Bulgarian-Hungarian human-trafficking gang", said Hans Peter Doskozil, police chief in the Austrian state of Burgenland.
Austrian newspaper Kurier quoted a government official specialising in human trafficking as estimating that the people inside the lorry would have paid smugglers a total of around 700,000 euros ($785,000) for their entire journey.
The public prosecutor in Burgenland, Johann Fuchs, said he would likely seek to have the suspects extradited to Austria and that they could be tried for murder.
The discovery of the truck in Austria, a rare occurrence on land in a prosperous country when so many migrants have died at sea, provoked a wave of horror across Europe.
Austrian daily Oesterreich calculated Saturday that the 71 were crammed into 15 square metres (160 square feet) and would have asphyxiated in little over an hour once they were shut inside with no air, the children suffocating first.
Austrian motorway maintenance workers alerted police after noticing "decomposing body fluids" dripping from the truck, Doskozil said.
The police were confronted by an overpowering stench and a mass of tangled limbs, and forensics experts worked all night to clear out the vehicle. TV images showed flies buzzing around the back of the vehicle.
'We don't know where to go'
"If the stink from our car parks gets stronger perhaps we will finally understand, not just in Austria... that it is time to create safe routes to Europe, fast registration and a swift and a fair sharing out (of migrants)," said Amnesty International's Austrian chief Heinz Patzelt.
In Germany, where Chancellor Angela Merkel was booed this week by far-right protestors, a demonstration to show solidarity with refugees was planned in Dresden on Saturday.
Austrian Chancellor Werner Faymann and French President Francois Hollande called during a phone call for a "unified European system of asylum and a common migration policy".
The victims in Austria were likely among the more than 100,000 people to have trekked up through the western Balkans into EU member Hungary this year.
From Hungary, which is laying a barbed-wire barrier along its border with Serbia to try to keep migrants out, many attempt to make it to richer nations like Germany and Sweden.
On Friday, Hungary's government proposed stiffer penalties for border jumpers and people smugglers, including a three-year jail term for those caught breaching the new fence.
"We passed by sea. And the sea was just a game playing with our lives," said Lashkari, a 30-year-old Afghan picked up by Hungarian border police Thursday after travelling for 30 days.
"I don't think we've reached our final destination yet because after this we don't know where do we go," he told AFP.