Huge traffic jams stretching back 50 kilometres (30 miles) built up along the Austro-Hungarian border on Monday as Austrian police stepped up security checks in the hunt for people smugglers.
The measure was put in place on Sunday, 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 at 1830 GMT, 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.
140,000 migrants arriving via Serbia
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, 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.
93 traffickers arrested in Upper Austria
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.
Pressure to take their fair share
Austria's Interior Minister Johanna Mikl-Leitner on Monday proposed cutting EU funding for member states that don't take in their fair share of migrants.
"You can't just cherry-pick within the European community," she said of countries which have resisted accepting many asylum-seekers as the European Union struggles to absorb its biggest influx of migrants and refugees since World War II.
One way to put pressure on those states which don't take their fair share, would be "scrapping or cutting financial support", Mikl-Leitner told German public broadcaster ZDF.
"Either Europe fails because of the refugee issue" or it will emerge stronger through a "fair" distribution of refugees, she said.
Austria has tightened controls on motorways near the Hungarian border, where an abandoned lorry was found last week containing 71 dead migrants, four of them children.
The minister said that building a fence, as Hungary is doing on its border with Serbia, was no solution, and the idea that such a move would discourage refugees was an "illusion".
Germany -- which expects to receive 800,000 asylum seekers this year, four times more than in 2014 and more than any other EU country -- has called for a fairer distribution of refugees between EU countries.
Thousands catch trains to Vienna, Munich and Berlin
Several hundred migrants stuck for days in makeshift refugee camps at train stations in Budapest were on Monday allowed to board trains headed for Austria and Germany, an AFP correspondent at the scene said.
Hungarian police had previously prevented as many as 2,000 migrants from leaving the station because they had no legal papers.
But on Monday there were no security forces present as the refugees, many from Syria, rushed to get on trains leaving for Vienna, Munich and Berlin.
People were running along the platform to catch an Austria-bound train scheduled to leave at 1110 GMT, with some helping to lift a woman in a wheelchair into a carriage.
There were confusing scenes as a Hungarian railway employee initially refused to allow the train to leave, saying it was packed beyond capacity and some people did not have valid tickets.
But the train eventually departed with a 20-minute delay.
However, an Austrian police officer at the station told AFP the migrants would be stopped once they crossed into Austria.