Warning: No-Groping Zone

They delivered a cold stare, then a sharp reprimand and finally gave a rousing shout for Roque Jose Santos to get out of their train car.

"This one's just for women, don't you know?" one female passenger cried as the 66-year-old music technician scuttled out of the compartment where, under Brazilian law, no men are allowed.

Last week, Rio de Janeiro became the world's third major city to bring women-only cars to its commuting trains and subways, joining Tokyo and Mexico City. A new law mandates a separate car for women during rush hour, marked on the outside by a pink-striped sticker or a sign with the symbol of a woman.

Many women have welcomed the law as a relief from the groping and sexual harassment they regularly experience in the packed cars. "Men think it's extremely normal to do this. They don't feel guilty at all," says Monica Aranjo Neves, 34, an administrative assistant who has been groped on several occasions. "We have to go to work, then take care of everything at home, and we shouldn't have to deal with this on the train."

Although transportation security will intervene, female passengers have taken it upon themselves to keep out invading men. While most men duck out quickly once they've realized their mistake, some are adamant about staying put, infuriated by what they consider to be a discriminatory measure.

"I'm against any form of inequality and segregation," says Paulo Vitor Matia, 18, a student. "They're acting as if women aren't capable of defending themselves, and they're labeling all men as abusers." Within the first week of the law, police have registered at least two incidents of men who claim to have been violently ejected from the women-only cars by security officers.

Proponents of the law argue that it was popular complaint that prompted the measure, gauged by calls received on a citizen's hotline. "It might seem like a law from another century. But unfortunately, the behavior of some Brazilian men is from another century," says a spokesperson for Representative Jorge Picciani, president of Rio de Janeiro State's Legislative Assembly, who authored the law. "It's a very old problem. Our grandmothers used to carry crochet needles on the trains to defend themselves from abuses. Whoever rides the trains knows how it is."

According to an online poll conducted last February by O Globo, a Brazilian newspaper, 67 percent of those polled were in favor of the measure. The widespread nature of the problem has even reached Orkut, the social networking website popular amongst Brazilians. An online community dedicated to "those who enjoy pressing up against women on crowded buses and trains" boasts 130 members and encourages them to leave descriptions of their encounters on the site.

Nevertheless, some believe the problem has little to do with the repellent intentions of Brazilian men -- and everything to do with an underfunded, overcrowded transportation system. "This happens in any crowd. The proximity of bodies creates a sense of permissiveness," says Andreia Maciel Garcia, 34, a theater professor at UniverCidade in Rio. "The issue is that these trains are too crowded, and the service provided is insufficient to give enough room for passengers to be comfortable."

Advocacy groups, moreover, consider the measure to be a serious step backwards for women's rights in Brazil -- and have begun a campaign to declare the state law unconstitutional. Women's advocates acknowledge that harassment is pervasive and grossly under-reported, but they believe that the law is an illegal form of discrimination that will only legitimize the abuses that still occur. Calling the creation of the law an act of "political opportunism," Rogeria Peixinho, coordinator of the Campaign for Brazilian Women, says that the proposal was constructed without any consultation of women's groups or organizations, either within or outside of the government.