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Twitter: Gavin Anderson

A stand-off between police and protesters is took place during the May Day protests in Oakland, Califoria, where police reportedly used tear gas, pepper spray and flashbang grenades to scatter demonstrators.

Flashbang grenade just went off. Pandemonium, tweeted Mother Jones' John Harkinson. I just got teargassed.

At the height of the confrontation, Occupy protesters shouted the whole world is watching! and please go home! to police officers, who, holding batons and standing in a tight line, blocked a busy thoroughfare from the large crowd of people trying to march through. In some places, police erected barricades as police vans and ambulances waited behind.

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Twitter: Josh Harkinson

More than any other city involved in the Occupy movement, Oakland has been rife with clashes between police and demonstrators. Occupy Oakland protestors became too familiar with flashbangs in October, when Iraq war veteran Scott Oslen was hit in the head with a grenade that many witness say was thrown directly at him and other protestors.

The impact fractured Olsen's scull and put him in a coma, from which he eventually recovered. Oslen, who has filed a claim against the Oakland police department, attended the protests on Tuesday, but only as an observer, according to Mother Jones.

During the May Day protests, police fired approximately two cans of tear gas at protesters at the intersection of 14th street and Broadway after a scuffle broke out, according to San Leandro resident Buddy Roark, 24, who was at the demonstrations.

According to Roark, there were about 200 demonstrators at the intersection.

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Twitter: Robert Salonga - Tear gas at 14th and Broadway

After police arrested an unknown number of people, including one woman who rode her bike through the march, protesters regrouped at Frank Ogawa Plaza. From there, many marched on to City Hall, while a number of police officers stayed behind at 14th street.

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Twitter: Jean-Philippe Dobrin - A protestor hit in the head by a police baton.

A spokeswoman for Oakland's mayor's office said that no one was immediately available for comment. The Oakland police department could not be reached for comment.

Ryan Villarreal contributed to this report.