UPDATED 7.51PM: About 2500-3000 refugees have entered Austria from Hungary in the past few hours, after Hungary began taking people in buses from Budapest to the border.

Police say they are waiting for 17 or 18 double-decker buses to be able to take people to Vienna, and maybe also towards Germany.

One train carrying around 400 refugees westwards has already left the Austrian town of Nickelsdorf on the border with Hungary.

Austria and Germany have agreed to receive thousands of refugees due to arrive at the Hungarian border, Austria's Chancellor Werner Faymann says.

Faymann told Austria's APA news agency that he had informed Hungary's Prime Minister Viktor Orban of the decision "in consultation" with German Chancellor Angela Merkel.

He said it was motivated by "the current emergency at the Hungarian border".

Hungarian officials on Friday said they were laying on buses to ferry desperate refugees who had set off on foot for the Austrian border after being stranded for days in Budapest's main railway station.

Buses will be sent to the Keleti station in Budapest, which has been besieged by more than 2000 refugees trying to get to Austria by train.

Buses will also be sent to pick up about 600 people who broke off from the crowd at the station and started walking along a motorway to the border 160 kilometres away.

Earlier on Friday, Hungary passed harsh laws aiming to discourage refugees from crossing the border illegally, while Greece agreed to build a large reception centre for refugees.

Starting on September 15 border trespassing becomes a criminal act in Hungary, smuggling people will be punishable by 20 years in prison, and transit zones designed to funnel migrants toward registration centres on the border will be set up.

Nearly all the refugees come via the so-called Balkan route crossing Turkey, the Aegean Sea to Greece, Macedonia and Serbia, where they enter Hungary and Europe's border-free Schengen zone.

Most hope to claim asylum in Germany, but under EU law, the country that first registers a refugee processes his or her asylum claim.

A new registration centre in Piraeus near Athens, where refugees are ferried from the islands at a rate of more than 4000 a day, will be run with the help of EU organisations such as border agency Frontex.

It will identify, register and fingerprint refugees, as well as assess whether they have a genuine asylum claim.

Those who do not are to be sent back to their country of origin.

"If we allow everyone in, that is the end of Europe. We may one morning wake up and realise that we are in the minority on our own continent."