Anti-Islam protesters -- some of them heavily armed -- faced off against religious rights demonstrators outside a US mosque, in a tense but peaceful standoff.

About 200 demonstrators from each group stood outside the mosque in Phoenix, Arizona, where a biker crew had said they would hold a Mohammed cartoon contest.

Dozens of officers lined up between the two sets of protesters, using yellow police tape to separate them near the gates of the Islamic Community Center mosque in north Phoenix.

"Stop Islam" was among the slogans on placards held by bearded bikers, one of whom wore a T-shirt reading "Support Your Local White Boy" and had a shouting match with rival demonstrators across the police lines.

Several anti-Islam protesters were heavily armed -- some with AR-15 assault rifles -- and wore military fatigues. Arizona has some of the laxest gun laws in the United States.

But after several hours of tension, the standoff eased -- though officers kept the two groups apart.

Organizers of the anti-Islam protests said on their Facebook page that it was in response to a recent failed attack in Texas where two gunmen were shot dead before they could storm an event including a Mohammed cartoon drawing competition.

"Everyone is encouraged to bring American flags and any message that you would like to send to the known acquaintances of the 2 gunman," who frequented the Phoenix mosque, the Facebook page said.

The anti-Islam protest's organizer, former Marine Jon Ritzheimer, told CNN before the Friday night demonstration: "I think the whole thing, the cartoon contest especially, I think it's stupid and ridiculous.

"But it's what needs to take place in order to expose the true colors of Islam," he added.