By Marie-Louise Gumuchian and Alex Fraser

BOSTON, England (Reuters) - Above the Polish Mini Mart food and drink store in the small eastern town of Boston, six English flags, the red cross of St George, flutter in the wind in front of an apartment window.

More flags are displayed behind the glass, along with placards, all bearing a clear message for Thursday's British referendum on EU membership: "Vote Leave".

Whether Britain decides to leave or to stay, many among Boston's eastern European migrants, including its many Poles, are worried about anti-EU feeling stirred in a tense, often emotional campaign.

"People fear attitudes will change," said Patrycja Walentynowicz, co-founder of Lincs PL which provides translation and other services to her fellow Poles in Boston.

"Since talk of a Brexit can sense tension between foreigners and Britons, who have shown their reluctance (towards immigration) and are demonstrating it more openly."

Immigration has become an emotive issue, with the "Out" camp's focus on it criticized by pro-EU campaigners as divisive. But its key argument has also struck a chord with many who say the arrivals are straining public services.

The "Leave" campaign has argued migrants are partly responsible for long waiting lists for social housing and difficulties getting doctor appointments. The rival "Remain" campaign counters that EU migrants as a whole pay more in taxes and labor charges than they cost the economy.

In Boston and its relatively eurosceptic Lincolnshire county, according to a survey, the issue is prominent. A January report by the right-leaning Policy Exchange named Boston "the least integrated place" in Britain.

A small town near the eastern coast, Boston has seen several streets transformed with eastern Europeans making use of the bloc's right to free movement to come work in nearby fields and businesses.

A 2011 census showed the proportion of Boston's foreign-born residents jumped to 15.1 percent from 3.1 percent in 10 years, many from Poland and Lithuania, which joined the EU in 2004, and put the town's population at around 65,000. Those numbers are expected to have increased since.

Along Boston's West street, the change is visible with Polish stores and restaurants beside Lithuanian businesses. At St Mary's church, three of four Sunday masses are in Polish.

"People are worried about what will happen if Britain votes to leave," Polish priest Stanislaw Kowalski said.

Walentynowcz's colleague Iza Paczkowska said Poles were increasingly opting for periodic tenancies instead of fixed term contracts in light of the uncertainty.

"We don't know how native English people are going to respond to us; if they're going to carry on...being nice and polite or whether they are just going to say 'We voted for you to leave so why are you still here'," she said. "That would be the most common worry. People are just preparing for leaving."

No Poles interviewed by Reuters noted aggression towards migrants ahead of the vote. Many declined to comment.

"What I fear is that social attitudes may change in a radical, nationalist direction," said Karol Sokolowski, who works at a Polish family restaurant.

"I know Britons aren't like that...But the publicizing of this (Brexit) ...can lead to some kind of, I can't say racism, but prejudice. This can happen, I am a bit afraid of this."

Jonathan Noble, a councillor of the anti-EU UK Independence Party (UKIP), said those working and settled with families "had nothing to fear".

"People...are concerned about not immigration per se," he said. "But the amount of immigration."

(Reporting By Marie-Louise Gumuchian and Alex Fraser)