Events on the Serbia-Hungarian border reveal "a dark side of the European character", David Miliband said.
The chief of the International Rescue Committee in New York was referring to scenes where police tear-gassed crowds of refugees including women and children.
"Anyone with an ounce of morality feels appalled by what's happening in parts of Europe," Mr Miliband told The Associated Press.
The former foreign secretary praised German leadership in tackling Europe's migrant crisis.
But he said that Hungary's decision to erect a razor-wire fence to stop the influx of refugees was "misguided and short-sighted, and when it's combined with bullyboy tactics it's obviously appalling".
Serbian doctors say two people were seriously injured and up to 300 have sought medical help after Hungarian police used tear gas and water cannons to stop migrants from entering Hungary.
In the last few months, Hungary has become a main entry point and bottleneck into the European Union for migrants, many of them war refugees from Syria, Iraq and elsewhere in the Middle East.
More than 200,000 migrants have entered Hungary so far in 2015, nearly all by walking across the southern border with Serbia, as they make their way to Germany or other wealthy Western European nations.
Mr Miliband's comments came two days after the 28-nation EU failed to come up with a united immigration policy at a contentious meeting in Brussels.
Ministers did agree to share responsibility for 40,000 people seeking refuge in overwhelmed Italy and Greece and spoke hopefully of reaching an eventual deal on which EU nations would take 120,000 more refugees, including some from Hungary.
The IRC boss called the EU failure "disappointing in all kinds of ways," but said Europe has no choice but to find another solution.
"Kicking this can down the road, kicking these people down the road is obviously no answer," he said.