The report, based on the testimony of migrants walking through from Bulgaria into Serbia, cites numerous cases of alleged police beatings and extortion.
Many migrants opt to travel through the Balkans, afraid of the sea-crossing from Turkey to the Greek islands.
The Bulgarian government has not yet responded to the allegations.
Hundreds of thousands of migrants, many from Syria, Africa and Afghanistan, have been making their way from Turkey to the Balkans in recent months, in a bid to reach Germany, Sweden and other EU states.
Collated by the Belgrade Centre for Human Rights and sponsored by Oxfam, the report highlights the dangers which await migrants both near the Turkish-Bulgarian border and later as they try to cross into Serbia.
Cases of alleged extortion by Bulgarian police, physical violence against migrants, and attacks by police dogs are described in detail, reports the BBC's Nick Thorpe.
The report findings are based on interviews conducted with over 100 refugees arriving from Bulgaria in the Serbian border town of Dimitrovgrad over a three-day period in October.
Stefano Baldini, Oxfam director for South East Europe, said: "In light of the reported abuses, the European Union has to intervene and take concrete action to protect basic human rights within its borders."
"These testimonies present a consistent picture of alleged incidents in Bulgaria," he said according to the UK's Press Association.
Last month an Afghan migrant was shot dead by a Bulgarian border guard after entering the country from Turkey.
Officials said he was killed by the ricochet of a bullet fired as a warning to the migrants.
UNHCR spokesman Boris Cheshirkov said the agency was "deeply shocked" by the incident and called on the Bulgarian authorities to commission an independent investigation into the death.
The Bulgarian authorities have been accused of brutality towards migrants in a new report sponsored by the British charity Oxfam.