Consistent reports of Bulgarian police abusing refugees - including a child aged 15 - and a number of alleged shootings have been uncovered by Oxfam.
The British charity called for European Union action and an independent investigation after at least one man was killed and others allegedly wounded by gunshot.
Some also claimed they were bitten by Bulgarian police dogs while fleeing the conflict in Afghanistan.
The Press Association heard first-hand accounts from teenagers, one aged 15 who suffered an allegedly broken nose.
Other young men displayed damaged hands and a badly bruised eye.
The victims blamed police for beating them with half-metre-long truncheons and branches during their perilous journey through the Bulgarian mountains, guided by people smugglers extorting thousands of euro from them.
Stefano Baldini, Oxfam director for South East Europe, said: "These testimonies present a consistent picture of alleged incidents in Bulgaria.
"In light of the reported abuses, the European Union has to intervene and take concrete action to protect basic human rights within its borders."
Weligol Alimjan, 20, a bridge-building mason from Afghanistan, suffered bad bruising and bleeding to his left eye after he was hit with a baton.
He said he fled Islamic State intimidation and would like to reach the UK, where human rights were respected.
He told the Press Association: "I left my home town thinking there was not this disturbance here and I realised that in Bulgaria I had a worse experience.
"I want people to know what is happening in the middle of the road."
People smugglers charge 600 to 2,000 euro (£425 - £1,415) for a perilous 45-mile (72km) journey to the heavily forested border with Serbia through the mountains, the last 10 miles (16km) on foot in the darkness, running the gauntlet of allegedly hostile police.
Despite that, there were many people at a centre across the border in Serbia with no physical signs of abuse.
Oxfam is to establish mobile protection units in Serbia to refer victims to other health services and document the attacks.
Mr Alimjan, who travelled with a group of friends through the autumnal landscape, said he was bound for wherever he could get asylum, wherever he could find work - with the UK top of his list.
"I like the UK, human rights are more respected there than in other countries," he said.
Hamayoon Noorhaman, 15, set out alone to walk from Afghanistan to the West after the Taliban intimidated him from school.
His nose was badly cut and may be broken after he said he was hit by a police officer wielding a branch.
He told the Press Association: "My life was very dangerous in Afghanistan because the Taliban wanted me to go to war but here it is OK, I was beaten but it is not the same."
There are separate unconfirmed reports of security force mistreatment of Syrians in Turkey as one of the largest mass migrations in recent history pours into Europe.
Migrants who make it through Bulgaria's border crossing with Serbia have described harrowing scenes of alleged police brutality for an Oxfam report called Safe Passage.
The interviews were conducted in the Serbian border town of Dimitrovgrad where up to 200 people cross from Bulgaria every day.
Accounts include:
:: A large group of children said they were threatened using guns and dogs by six Bulgarian police officers trying to force them back behind the Turkish border. While some escaped, 15 were detained and one shot in the leg.
:: A group of around 10 interviewees witnessed a police officer holding a gun to a refugee's forehead, while others lay on the ground, apparently unconscious. They fled in fear but police caught them and took valuables, food and water.
"Later, close to the Serbian border, police officers released dogs on them, and gun shots were heard," the interviewers were told. Seven people from the group went missing and they have not had contact with them since."
:: Two Afghan men said officers had shot at them to prevent their escape.
:: Another in his 20s said he was pistol-whipped by police.
On October 15, an Afghan refugee was shot dead near the Turkey-Bulgaria border, Oxfam said.
Mr Baldini added: "Refugees arriving in Europe must be treated with respect and they have the right to protection.
"They should not have to experience brutality and xenophobia."
The report said: "These allegations are largely consistent and include extortion, robbing, physical violence, use of weapons, threats of deportation and attacks by police dogs.
"All of the refugees interviewed, except those who had not had any contact with the police, reported ill-treatment in Bulgaria."
Almost half those registered in Serbia this year were in October alone and Oxfam is bracing itself for cold weather and the need to provide shelter against sub-zero temperatures.
The charity is putting a range of protection measures in place and has information boards close to border crossings to aid migrants.