His intervention also follows an attack on a Polish national by 20 men in Leeds which police have said was a racially aggravated assault.
Mr Juncker defended the free movement of European Union citizens - seen as a key factor in the vote to leave - as he denounced the killing of Arkadiusz Jozwik in Harlow last month.
Police are treating the attack as a possible hate crime and six teenagers, aged 15 and 16, have been arrested and released on bail.
Poles have been victims in a series of incidents since the UK voted to leave the EU in June, including the assault of two Polish men within hours of a vigil following Mr Jozwik’s death.
Amid the violence, Theresa May called the Polish prime minister Beata Szydlo to express her “deep regret” and to stress “hate crime has no place in UK society”.
Now Mr Juncker has intervened. At his state of the EU address to the European Parliament, he said: “We Europeans can never accept, never, Polish workers being harassed, beaten up or even murdered in the streets of Essex.”
He added: “The free movement of workers is as much a common European value as our fight against discrimination and racism.”
Meanwhile, Mr Juncker suggested that Britain cannot be a member of the European single market and regain full control over immigration from the continent.
West Yorkshire Police said the 28-year-old Leeds victim was seriously injured when he was kicked and punched by a group of up to 20 youths in Alliance Street, Armley.
The attack prompted a statement from the Polish Consulate in Manchester, saying it was the most serious of more than ten xenophobic incidents involving Polish people in the North of England that it has dealt with.
Consul General Lukasz Lutostanski said; “We are grateful to the police for a decisive response and promise to send additional patrols to the area of the crime scene. We will work together on solutions that will permanently increase the safety of Polish nationals.”
Police said the victim and a friend had been confronted by the group and assaulted in nearby Town Street shortly before the attack. The victim ran off along Wortley Road and was pursued by the group to Alliance Street.
Chief Superintendent Paul Money, Leeds District Commander, said: “I want to reassure people, particularly the local Polish community, that we will not tolerate hate incidents of this nature and will do everything we can to ensure the people responsible are brought to justice.
“Armley is an area with a strong sense of community across a population from a range of ethnicities and nationalities and I know that people living there will be as shocked and appalled as we are at this incident.
“I appreciate this attack will have understandably caused heightened concerns among eastern European communities but we are treating it as an isolated incident and not as an indicator of any ongoing hate issue in the area.”