Volunteers in a convoy of around 140 vehicles have set out from Vienna to cross into Hungary to distribute aid to refugees and bring them back to Austria.
Onlookers chanted: "Say it loud, say it clear, refugees are welcome here," as the activists - organised via Facebook and loaded with food, water, sanitary products, warm clothing and stuffed animals - set forth from outside a football stadium on Sunday.
People taking part ran the risk of violating laws on human trafficking, police said, but officers were on hand just to provide security and guide traffic.
The convoy aimed to cross into Hungary after stopping at the refugee reception centre in Nickelsdorf on the Hungarian border.
Kurto Wendt, an Austrian organiser of the campaign, told the Reuters news agency that he was not scared of being arrested in Hungary.
"Everyone would really be crazy if they arrest people who support what they have decided politically, namely to let people move on to Austria. The risk that we are taking is so small compared with what the refugees are going through," he said.
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Austria and Germany have thrown open their borders to the wave of refugees making their way north and west from the Middle East, Africa and elsewhere. Hungary has been letting the human tide move on after holding it up for days last week.
Asked why the group was acting if refugees could now board buses or trains for the Austrian border, Wendt said: "Ten children were hospitalised overnight. The people are hungry, with poor clothing. Every day that you don't get people to safety they can die, so we have to do this immediately."
After days of confrontation and chaos, Hungary deployed more than 100 buses overnight to take thousands of the refugees who had streamed there from southeast Europe to the Austrian frontier.
Al Jazeera's Mohammed Jamjoom, reporting from Nickelsdorf, where the convoy was headed, said every refugee he had spoken to "was glad they left the horrendous experience they said they had in Hungary these past few days".
Also on Sunday, Pope Francis called on every European parish, religious community, monastery and sanctuary to take in one refugee family, as thousands of people continued to stream into Germany via Austria.
The pope said the Vatican would open its doors to two refugee families, but provided few details as he addressed tens of thousands of people in St Peter's Square.
On Saturday, Finland's Prime Minister
offered his private home in northern Finland to refugees.