When graffiti artist Shepard Fairey turned his talents to US politics, his reward was international acclaim and a letter of thanks from Barack Obama. When he employed a similar tactic in Denmark, however, the response proved altogether less edifying.

Last weekend, Fairey – creator of the famous "Hope" poster that came to encapsulate Obama's 2008 presidential campaign – was beaten up after the opening of his exhibition at a Copenhagen gallery.

Earlier this month he was involved with a controversial mural that has enraged leftwing anarchists throughout the city.

"I have a black eye and a bruised rib," Fairey told the Guardian.

According to reports, 41-year-old Fairey and his colleague Romeo Trinidad were punched and kicked by at least two men outside the Kodboderne 18 nightclub in the early hours of last Saturday morning. Fairey claims the men called him "Obama illuminati" and ordered him to "go back to America".

The LA-based artist believes the attack was sparked by a misunderstanding over his mural commemorating the demolition of the legendary "Ungdomshuset" (youth house) at Jagtvej 69. The building, a long-term base for Copenhagen's leftwing community, was controversially demolished in 2007. In the intervening years it has become a potent symbol of the standoff between the establishment in Copenhagen and its radical fringe.

Fairey's installation, painted on a building adjacent to the vacant site, depicted a dove in flight above the word "peace" and the figure "69". But the mural appeared to reopen old wounds, with critics accusing Fairey of peddling government-funded propaganda.

"The city council is using the painting – directly or indirectly – to decorate the crater-like lot at Jagtvej 69," said local activist Eskil Andreas Halberg in a letter to Modkraft, a leftwing news website. "The art is being used politically to end the conflict in a certain way: 'we're all friends now, right?'"

Within a day of completion, the mural was vandalised by protesters, with graffiti sending messages of "no peace" and "go home, Yankee hipster". Fairey subsequently collaborated with former members of the 69 youth house to redecorate the lower half of the installation. His new version contains images of riot police and explosions, together with a new, more combative slogan: "Nothing forgotten, nothing forgiven".

Fairey explained that the original mural was organised by his Copenhagen gallery, V1, and was never intended as propaganda. "The media reported that it was commissioned by the city, which wasn't true," he told the Guardian.

"It looked to the people at 69 like I was cooperating with the authorities, making a propaganda piece to smooth over the wound." He added that he did not believe his attackers were affiliated to the 69 youth house.

Born to a middle-class family in Charleston, South Carolina, Fairey began his career within the skateboarding scene, designing boards and T-shirts before finding wider fame with his "Obey Giant" sticker campaign. In 2008 his unofficial Barack Obama campaign poster was hailed by the New Yorker's art critic Peter Schjeldahl as "the most efficacious American political illustration since 'Uncle Sam Wants You'."

While the Hope poster was never publicly endorsed by the Obama campaign, its subject sent Fairey a letter. "I would like to thank you for using your talent in support of my campaign," Obama wrote. "Your images have a profound effect on people, whether seen in a gallery or on a stop sign. I am privileged to be a part of your artwork and proud to have your support." Fairey's work now hangs in the Smithsonian, the Museum of Modern Art in New York, the National Portrait Gallery in Washington and the Victoria and Albert Museum in London. These days, he admitted, he has grown used to defending himself against accusations of selling out. "I think there are a lot of people who think that if you've done well, you've done a deal with the devil," he said.

The artist said he had not filed a police report following the attack in Copenhagen. "I did not know any of the people or get a great look at them, so it seemed pointless," he said.

"I'm not a huge fan of the cops anyway. The only thing I could see coming out of it was further media commentary like 'street artist whiner Shepard Fairey can't hold it down in a fight so he snitches to the cops'."