Footage has emerged of migrants being thrown bags of food at a Hungarian camp near the border with Serbia.
An Austrian woman who shot the video said the migrants were being treated like "animals". Human Rights Watch's emergency director said people were being held like "cattle in pens".
Hungary says it is investigating the scenes at the camp in Roszke.
Meanwhile, Central European ministers again rejected a mandatory quota system for sharing out migrant arrivals.
"We're convinced that as countries we should keep control over the number of those we are able to accept and then offer them support," Czech Foreign Minister Lubomir Zaoralek said at a press conference with his Hungarian, Polish and Slovak counterparts.
The European Commission, with Germany's backing, has proposed sharing out 160,000 asylum seekers a year between 23 of the EU's 28 members.
The Central European states had already rejected the plan, even though they would take in far fewer refugees than Germany if the EU backs it. European Council President Donald Tusk has said he will call an emergency summit later this month if a solution is not found soon.
In recent weeks, tens of thousands of migrants have been desperately trying to make their way to Europe from war-torn Syria and Libya. Many travel through Hungary to Germany, Austria and Sweden - wealthier EU nations with more liberal asylum laws.
BBC correspondents and producers covering the migrant crisis on Twitter
- Anna Holligan at Roszke on the Hungarian border: "#Hungarian government responds to shocking scenes, 'we're investigating'"
- Manveen Rana at Roszke: "A man held his baby out of the window shouting "oxygen, oxygen". A guard shouted at him to get back in again"
- Bethany Bell in Nickelsdorf, Austria: "Austrian army putting up tents at the border"
- Wietske Buremaat the Macedonia-Greek border: "Drilling a channel for a drinking water pipe for refugees at the border of Greece & Macedonia"
Hungary has become a key point on the journey. The footage comes from a camp at Roszke, where large numbers of migrants have built up.
It was filmed by Michaela Spritzendorfer, the wife of an Austrian Green party politician who was delivering aid to the camp, and Klaus Kufner, a journalist and activist.
"These people have been on a terrible tour for three months," said Michaela Spritzendorfer.
"Most of them have been across the sea now and on the boat and through the forest and they've gone through terrible things and we, as Europe, we keep them there in camps like animals," she told the BBC. "It's really a responsibility of European politicians to open the borders now."
Human Rights Watch said migrants were being kept in "abysmal" conditions at two detention centres in Roszke, lacking food and medical care. The group quoted two migrants who described the conditions as only fit for animals.
At the scene: Anna Holligan, BBC News, Roszke
The Hungarian refugee camps have become humiliating holding zones for the thousands trying to cross the country's borders. Journalists are banned from entering, but images shared by human rights groups and refugees are disturbing.
The Hungarian government has not yet commented, but the images will fuel the allegations that Hungary is failing to meet the minimum standards for the treatment of migrants, as laid out in the European Convention on Human Rights.
The Council of Europe has reminded member states that people should not be treated like prisoners.
Many of the people I've spoken to, from Raqqa, Idlib and Homs have become numb to violence in Syria, but their treatment in what is supposed to be a place of refuge is hard to bear.
There is also a bottleneck at Hungary's border with Austria. Officials said about 8,000 people had crossed into Austria at Nickelsdorf on Thursday and a similar number were expected on Friday.
Existing shelters in the area are full and the army is putting up tents, the BBC's Bethany Bell reports. Exhausted men, women and children are everywhere, some even sleeping on the manicured gardens of Nickelsdorf's neat houses.
With no buses running early in the morning, and just one packed train departing, some have started walking along the motorway towards Vienna, which police have closed to traffic.
On Wednesday, the Hungarian army started military exercises to prepare for a possible future role in guarding the border and stemming the flow of people.
A new razor-wire barrier is also being constructed along the country's border with Serbia, and Hungary said on Friday that it was increasing the number of troops deployed to build it.
A UNHCR spokesman said the agency was "closely following" Hungary's use of soldiers and expected the authorities "to respect rights of refugees whether they are the police or army".
A note on terminology: The BBC uses the term migrant to refer to all people on the move who have yet to complete the legal process of claiming asylum. This group includes people fleeing war-torn countries such as Syria, who are likely to be granted refugee status, as well as people who are seeking jobs and better lives, who governments are likely to rule are economic migrants.