Austrian police have intercepted a migrant truck packed with 26 people, including three severely dehydrated young children, barely two days after the bodies of 71 migrants were found inside a truck parked on one of the country's highways.

Key points: Austrian police intercept truck containing 26 migrants, arrest driver

Austrian police intercept truck containing 26 migrants, arrest driver Four men charged over deaths of 71 migrants found in abandoned truck in Austria

Four men charged over deaths of 71 migrants found in abandoned truck in Austria Vigil held outside train station where many migrants are sheltering

Vigil held outside train station where many migrants are sheltering Between 2,000 and 4,000 demonstrate in Germany

Police said the vehicle was pulled over at Braunau am Inn, close to the German border, adding that the migrants had been en route to Germany.

"The three small children had to be hospitalised ... because of the extremely bad state of their health. Doctors said they were treated for severe dehydration," police said of the recent discovery.

"If the journey had continued the situation could probably have become critical."

The Romanian driver, who had refused to pull over for a routine check, was arrested.

The discovery of the truck in Austria has again highlighted the dangers faced by people fleeing conflict and hardship in the Middle East and Africa, especially those who have put their fate in the hands of profit-hungry people smugglers.

Four charged, remanded over death of 71 migrants

Meanwhile, four suspected human traffickers have appeared in court over the deaths of the 71 migrants found dead on Thursday.

The defendants — three Bulgarians and one Afghan — told the court in Hungary, where the truck originated, that they were innocent.

The men were remanded in custody until September 29 in Kecskemet, a town near the Serbian border.

Forensic officers stand behind the truck containing a large number of dead migrants by a highway in eastern Austria. ( AFP: Dieter Nagl )

Police believe the suspects are low-ranking members of one of the numerous, unscrupulous people-smuggling gangs that transport migrants in return for sometimes exorbitant amounts of money.

Autopsies are still being carried out but police believe the victims, believed to be mainly refugees fleeing the war in Syria, may have been dead for up to two days.

Austria borders Hungary, which has seen more than 140,000 migrants enter from Serbia this year, most of whom then seek to travel onwards to western European countries like Germany and Sweden via Austria.

Hungary's defence minister said a razor-wire barrier along the border with Serbia, aimed at keeping out migrants, was completed ahead of an end of August deadline.

A four-metre-high fence is due to follow and is already being built by the Hungarian army which will "also provide a defence against illegal border-crossers," the defence ministry said.

Anti-racism protesters in Dresden chanted: "Say it loud, say it clear, refugees are welcome here." ( AFP: Robert Michael )

Hungarians hold vigil for dead migrants

Despite their government's hard line on migrants, a group of Hungarians held a vigil for the 71 truck victims outside Budapest's main train station, where thousands of migrants have been sheltering for weeks.

Other Europeans also indicated their support with between 2,000 and 4,000 people demonstrating in the eastern German city of Dresden on Saturday in solidarity with the migrants.

"Say it loud, say it clear, refugees are welcome here," they chanted.

A poll by a local broadcaster showed 60 per cent of Germans agreed with the country's chancellor Angela Merkel that Europe's biggest economy can accommodate the new arrivals.

Germany is expecting to receive a record number of 800,000 requests for asylum this year, four times higher than in 2014.

"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," UN secretary-general Ban Ki Moon said.

A woman holds a child at a transit zone in front of the Eastern railway station in Budapest. ( AFP: Attila Kisbenedek )

Reuters/AFP