Huge traffic jams stretching back 50 kilometres have built up along the Austro-Hungarian border as Austrian police step up security checks in the hunt for people smugglers.
The measure was put in place on Sunday (local time), three days after an abandoned lorry containing 71 dead migrants, four of them children, was discovered on a motorway near the Hungarian border.
Since the operation began, more than 200 migrants have been picked up, and another five people have detained on suspicion of smuggling, officials said.
"The main aim is to target smuggling gangs. What is happening here are controls conducted by traffic police and security forces – these are not border controls," said police spokesman Helmut Marban.
As part of the clampdown, carried out in close collaboration with the Hungarian, Slovakian and German authorities, police are stopping trucks, vans and cars in an effort to catch those trying to make money from people fleeing war and persecution.
Shortly after the checks began, officers discovered 12 migrants – nine adults and three children – crammed inside a minivan carrying a French number plate
The group was believed to be predominantly from Syria, media reports said.
The driver, whose nationality was not immediately clear, was arrested, the Austrian authorities said.
The new security checks caused a huge build-up of traffic overnight, and by early Monday, there were tailbacks of up to 30 kilometres along the main motorway connecting Budapest to Vienna.
Around noon on Monday, there were 50-kilometre queues near the Austrian border town of Nickelsdorf, according to local mayor Gerhard Zapfl.
Many migrants picked up in Austria have already trekked on foot through four or five countries before they reach EU member Hungary, where smugglers pack them into vehicles without food or water, promising to transport them to richer European states like Germany or Sweden.
So far this year, the Hungarian government has reported more than 140,000 migrants entering the country from Serbia.
Since Hungary is in the visa-free Schengen zone, onwards travel has been relatively easy.
But Austria's reinforced border units and its tightened security checks are likely to make the process of entering the country illegally much harder.
This year alone, police in Upper Austria have arrested 93 human traffickers transporting a total of 1,630 migrants.
Hungarian police said on Sunday that a fifth suspected human trafficker had been arrested over Thursday's gruesome discovery of the 71 decomposing corpses.