By Paul Ingram

PHOENIX (Reuters) - More than 200 protesters, some armed, berated Islam and its Prophet Mohammed outside an Arizona mosque on Friday in a provocative protest that was denounced by counterprotesters shouting "Go home, Nazis," weeks after an anti-Muslim event in Texas came under attack by two gunmen.

The anti-Muslim event outside the Islamic Community Center of Phoenix was organized by an Iraq war veteran who posted photos of himself online wearing a T-shirt with a crude slogan denigrating Islam and waving the U.S. flag.

As the event got under way, demonstrators on both sides screamed obscenities at each other as police in riot gear swiftly separated the two groups, each with about 250 people, using police tape and barricades.

"This is in response to the recent attack in Texas," organizer Jon Ritzheimer wrote on his Facebook page announcing the event at a mosque targeted in part because the two Texas gunmen had worshipped there.

More than 900 people responded on the event's Facebook page that they would attend, and police expanded their presence in the evening in anticipation of growing crowds. Officers with riot helmets and gas masks formed a cordon for several blocks.

Among the anti-Islam protesters, some of whom called Islam a "religion of murderers," more than a dozen men in military clothing carried semi-automatic weapons. Others waved copies of caricatures of the Prophet Mohammad drawn at the Texas event.

By late Friday night, virtually all the protesters and police had left the area with no reports of violent flare-ups or arrests.

Depictions of Mohammad, which many Muslims view as blasphemous, have been a flashpoint for violence in Europe and the United States in recent months where those displaying or creating such images have been targeted by militants.

Anti-Muslim groups have been active in the United States, buying ads and staging demonstrations characterizing Islam as violent, often citing the murderous brutality of Islamic State militants in Iraq and Syria.

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'LOVE YOUR NEIGHBOR'

The Phoenix mosque targeted on Friday has condemned such violence and held a series of sermons at Friday prayers last year by an imam who criticized militant Islamist groups such as Islamic State, al Qaeda and Nigeria's Boko Haram.

The president of the center had urged worshippers not to engage with the demonstrators.

"We should remind ourselves that we do not match wrongness with wrongness, but with grace and mercy and goodness," Usama Shami told worshippers during Friday prayers.

While some counter-protesters outside the mosque responded to the anti-Islam protest with obscenities, others followed his advice and chanted "Love your neighbor."

Todd Green, a religion professor at Luther College in Iowa who studies Islamophobia, said that the brutal acts committed by Islamic State and other militant groups have colored many Americans' impressions of Muslims.

"Almost two-thirds of Americans don't know a Muslim," Green said. "What they know is ISIS, al Qaeda, and Charlie Hebdo," referencing the January attack on the Paris office of the satirical magazine Charlie Hebdo that left 12 people dead over anger at the magazine's cartoons featuring the Prophet.

In a similar incident, a pair of gunmen on May 3 opened fire near Dallas outside an exhibit of cartoons featuring the Prophet Mohammad and were shot dead by police.

Leaders of the Phoenix Muslim community confirmed both gunmen had attended the mosque targeted in Friday's demonstration.

U.S. officials are investigating claims that the Texas gunmen had ties to the Islamic State, but said they had not established a firm connection.

'EPIDEMIC OF ANTI-ISLAMIC SENTIMENT'

The Department of Homeland Security spoke with state and local law enforcement and monitored the situation in Phoenix, White House spokesman Josh Earnest said.

Ritzheimer, the main organizer of the demonstration, said the point was "to expose the true colors of Islam."

"True Islam is terrorism. Yes, the ones that are out committing these atrocities and stuff, they are following the book as it's written," Ritzheimer told CNN.

Ritzheimer was a staff sergeant in the Marine Reserve and was deployed to Iraq twice, in 2005 and 2008, the Marine Corps said.

Anti-Islam activist Pamela Geller, who organized the Texas event, said she was not involved in the Phoenix demonstration.

The mosque is a former church near the city's international airport that can hold some 600 worshippers. The Phoenix area is home to tens of thousands of Muslims.

Friday's event is part of "an epidemic of anti-Islamic sentiment" that goes beyond protesting against extremism, said Imraan Siddiqi of the Arizona chapter of the Council on American-Islamic Relations.

"Don't mistake that, they're not saying they want to rid America of radical Islam, they are saying they want to rid America of Islam," Siddiqi said.

(Writing and additional reporting by Alistair Bell, Sharon Bernstein and Curtis Skinner; Additional reporting by Scott Malone in Boston, and Alex Dobuzinskis and Dan Whitcomb in Los Angeles; Editing by Cynthia Johnston, Lisa Shumaker and Nick Macfie)