Some 2,200 people had joined a social media campaign in Austria by Friday afternoon to organise a convoy of private cars and vans on Sunday to help pick up hundreds of migrants stranded in Hungary.
"The Austrian government and the EU stand by idly and watch as people on the streets of Budapest -- without any appropriate supplies -- have to endure appalling conditions," organisers of the citizen initiative wrote on the Facebook page.
"That's why we are intervening and starting a convoy of buses and cars to bring the refugees to safety."
Campaigners have called on private drivers of cars and vans to meet in Vienna on Sunday morning and to head to Hungary in order to bring as many migrants and refugees as possible to Austria or Germany.
The organisers said they were reacting to the decision by Hungarian authorities to suspend rail links to western Europe as thousands of migrants have crowded its trains stations.
Four Austrian activists were arrested Friday in Budapest after allegedly planning to drive migrants to Austria, which is a crime in Hungary that carries up to five years in prison.
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Hungary's foreign affairs minister Peter Szijjarto said the activists would be promptly released.
"We find ourselves in a very difficult circumstance. We don't need people coming here and inflaming the situation," he added.
Austrian authorities affirmed on Thursday that aiding illegal immigration is against the law and carries an up to 5,000-euro ($5,500)fine.
The activists, citing the precedent of a train hired by Austria's national rail company to carry migrants, noted that they had arranged for "legal assistance" for participants.
Hungary has in recent months joined Italy and Greece as a "frontline" state in Europe's migrant crisis, with 50,000 people trekking up the western Balkans and entering the country in August alone.
A record 3,300 migrants crossed into Hungary on Thursday, according to the latest figures from the UN refugee agency.
The right-wing government headed by Prime Minister Viktor Orban has responded to the influx by erecting a controversial razor-wire barrier along its 175-kilometre (110-mile) border with Serbia.