Hungarian border police are systematically abusing migrants, through beatings, using tear gas and dogs and confiscating shoes in cold weather, according to Doctors Without Borders (MSF).
But the charges were swiftly denied by a government spokesman, who said Budapest "utterly rejects" the allegations.
In an interview with AFP, Christopher Stokes, one of MSF's directors, said the abuse appeared to be a way of deterring people from entering the country.
"This doesn't seem to be the behavior of one or two policemen that fly off the handle," he said.
"It seems to be much more systematic and routine and organized."
Last week, Stokes visited locations near Subotica, a town in northern Serbia close to the Hungarian border, as well as MSF's facilities in Belgrade.
Although he said he had not himself witnessed any abuse, he interviewed "dozens" of migrants who had tried to enter Hungary in recent weeks and months and who were examined by MSF.
Migrants "get a kind of welcome package, if I could say cynically, from the border police that seems to consist of beatings and tear gas," Stokes said.
Police also "have people lie on the floor and walk on them and run on them with their boots," he said, with injuries such as bruising and cuts that were "pretty consistent" with such claims.
Last year, injuries treated by MSF tended to be "more open wounds and broken bones," Stokes said.
Now "there seems to be a tendency to hit them on the muscles, on other parts of the body, to create less obvious or visible marks but they are still detectable," he said.
Since the start of the year, MSF has also noticed police using the cold weather "as an element of extra punishment" -- confiscating shoes and other items of clothing and making people stand and walk in the snow.
They have also treated people with marks apparently caused by muzzled dogs pressing against necks.
"We are used to treating people who have suffered violence, and the stories we are hearing match with what we are seeing in our clinics," Stokes said.
"It seems to be to deter them from coming again so that people will pass on the message: don't come into Hungary."