The news of the arrests came as Libyan rescue workers recovered 76 bodies from yet another capsized boat in the Mediterranean crammed with people fleeing conflict and hardship in the Middle East and Africa.
The United Nations refugee agency, UNHCR, said as many as 200 people on two boats were feared dead near the western port of Zuwara.
Many of the victims were of Arab or African origin, a Pakistani teenager who survived by clinging to wreckage for nine hours told AFP.
- Likely suffocation -
Austrian police said the truck victims were likely fleeing the war in Syria and included a toddler and three young boys.
"Among these 71 people, there were 59 men, eight women and four children including a young girl one or two years old and three boys aged eight, nine or 10," police spokesman Hans Peter Doskozil told a news conference.
He said the time and cause of death still had to be determined but there was a "certain probability" they had suffocated.
Doskozil said those arrested included the owner of the vehicle and two drivers, and were likely "low-ranking members... of a Bulgarian-Hungarian human-trafficking gang".
A court on Saturday is to decide whether they can be detained beyond an initial 72-hour period.
Austria's public prosecutor Johann Fuchs said he would likely seek to have the suspects extradited.
On Friday evening, Hungarians held a vigil for the victims outside Budapest's main train station, where thousands of migrants have been sheltering for weeks.
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.
Television images showed flies buzzing around the back of the vehicle in the baking sun.
- 'Who will stop this madness?' -
Austrian newspaper Kurier carried a black front page with the headline: "Who will stop this madness?"
German Chancellor Angela Merkel, in Austria Thursday for a summit with Balkan leaders on the migrant crisis, said those present were "shaken" by the news, which underscored the need to tackle the issue "quickly".
On Friday, she said that EU leaders could hold a special summit on the crisis, "to take certain decisions"
So far European Union leaders have struggled to speak with one voice on the situation, with many governments fearful of being seen as too accommodating to the newcomers given widespread anti-migrant sentiments.
On Friday, a German court on Friday reinstated a weekend ban on public gatherings in an eastern town hit by violent far right anti-migrant protests, overturning an earlier decision that found the restriction to be "illegal".
Explaining the ban in Heidenau, which included counter rallies in support of refugees, authorities said they could not ensure public safety.
- 300,000 people cross Med -
Over 2,500 men, women and children have drowned trying to reach Europe this year, out of more than 300,000 people estimated by the UN to have made the voyage.
In the latest disaster at sea, at least 76 people died Thursday after a boat carrying hundreds of migrants sank off the coast of Libya, a spokesman for the Libyan Red Crescent said, with 198 rescued.
The drownings came a day after a Swedish ship found 52 bodies in the hold of a wooden boat found drifting off Libya.
But the truck deaths in Austria have shown that even when migrants make it across the Mediterranean, their troubles are far from over, with many putting their fate in the hands of profit-hungry people smugglers.
"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.
- Stiffer penalties for border jumpers -
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 keep migrants out, many try 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 dont think we've reached our final destination yet because after this we don't know where do we go," he told AFP.