
The report, which was based on interviews with over 100 migrants arriving in Serbia from Bulgaria, said they suffered "extortion, robbing, physical violence, use of weapons, threats of deportation, and police dog attacks."
Bulgaria charges three men over dead migrants in lorry in Austria
Most said they were abused by police guarding the country's southeastern border with Turkey but some also said they were abused by authorities in Bulgaria's asylum shelters.
"The dramatic and shocking ill-treatment of people fleeing conflict and poverty is totally unacceptable, particularly in an EU member state," said Nikolina Milic of the Belgrade Centre for Human Rights, which conducted the survey on behalf of Oxfam.
"We are calling for an independent investigation into the incidents in Bulgaria, and we call on the Bulgarian government to condemn these human rights abuses in the strongest terms possible," she added.
The route through Bulgaria allows migrants to bypass a dangerous sea crossing from Turkey to the Greek islands.
Afghan migrant shot dead by Bulgarian border guards
But the poorest country in the European Union has installed a 30-kilometre (20-mile) razor-wire fence and deployed over 1,000 police at the 260-kilometre border, preventing migrants from crossing.
Last month, a 19-year-old Afghan migrant was shot and killed by police near the southern town of Sredets after entering illegally from Turkey.
The interior ministry said that the case was an accident caused by a ricochet from a police warning shot.
Police and migrants in stand-off near Turkey's Greek border
Human Rights Watch had already criticised Bulgaria for allegedly turning back migrants at the Turkish border with force, something which the authorities denied.
Asked about the new accusations, an interior ministry spokeswoman told AFP: "We have no comment for the time being."
"We have not had any signals of abuse in the refugee centres," refugee agency spokeswoman Avgustina Videva added.
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