CHICAGO (CBS) — Some protesters are planning to get a few things off their chests – in a very literal sense – at North Avenue Beach over the weekend.

At noon Sunday, in the middle of the Air and Water Show, women will take off their shirts and expose their breasts for National Go Topless Day.

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The protest event, which is now in its fourth year, proclaims that women have a constitutional right to go topless in public. It encourages equal opportunity by urging women to bare their breasts, and for bare-chested men to cover up with bikinis or bras.

Organizers say men should be legally required to cover up their chests as long as women are.

The topless protests are organized by the Raelian Church, a UFO religion that teaches life on earth was scientifically developed by a species of extraterrestrials who were mistaken for gods. The Raelian Church also gained notoriety for founding the human cloning company Clonaid in 1997, and claiming to have cloned a human being in 2002.

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But the Huffington Post reports a few hundred thousand people participate in the protest every year. It will be held in Chicago and numerous other U.S. cities on Aug. 21, and in Toronto and Vancouver in Canada on Aug. 28.

Going topless is not illegal for women everywhere in the country. The State of New York, for example, permits women to go topless in public areas.

But Chicago does not.

Under the city’s Municipal Code, it is illegal to “appear, bathe, sunbathe, walk or be in any public park, playground, beach or the waters adjacent thereto, or any school facility and the area adjacent thereto, or any municipal building and the areas adjacent thereto, or any public way within the City of Chicago” exposing “any portion of the breast at or below the upper edge of the areola thereof of any female person.”

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The prescribed penalty for indecent exposure – of the female breasts, or of the male or female buttocks or genitals – is a fine of $100 to $500.