Performances in front of Berlin's Brandenburg Gate are no rarity, but it isn't every day that tourists to the German capital can witness a fictional slave auction. Yet that was the spectacle on view on Friday, as activists from the group Gemeinsam für Afrika (Together for Africa) sold off "slaves" to illustrate a global problem that particularly affects the world's largest continent. Similar demonstrations were held in six other German cities from Leipzig to Stuttgart.

Gemeinsam für Afrika estimates that at least 40 million people around the world live in slave-like conditions, roughly half of whom are forced to do compulsory labor – a tragic historical record.

"We have more slaves today than at the high point of the slave trade," spokeswoman Susanne Anger told Deutsche Welle.

18 percent of all modern slaves come from Africa.

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"Africa represents a particular problem where slavery is concerned because there's such privation in so many countries," says Anger. "So people can be forced into conditions of slave labor all the more quickly. And they have a lots of things in Africa which we here want: rare ores, diamonds and all manner of precious stones."

These Afghan women are among the millions of slave laborers around the world

To the highest bidder

The three "slaves" confined in a makeshift pen at the Brandenburg Gate and sold off to a "bidder" wearing a golden mask represented actual cases of slavery in Europe.

Lisha, a 22-year-old from Nigeria, for instance, was forced to work as a prostitute in Germany, while 32-year-old Yvan Sagne from Cameroon ended up in tomato plantations in southern Italy.

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"When people are in serious need, for instance, when they come as refugees to us in Europe, it's relatively easy to compel them to do slave labor," Anger explains. "For example, people are brought to the plantations in Italy and Spain. That's where our tomatoes and our cucumbers come from, and they're produced under unspeakable conditions. People are forced to work for 12 to 14 hours a day."

Often, people are lured to work in Europe with the promise of free lodging and food only to see their already meager wages eaten away by charges for room and board, water and even "rental fees" for tools. This type of slavery is commonly known as "debt bondage" or "bonded labor."

Slave labor can play a role in a variety of common consumer goods

What's behind your vegetables

Although Africans are particularly likely to become slaves, modern slavery is a global phenomenon that comes in different forms and is part of the production process of a variety of goods in places around the planet.

Slave labor is used in the production of everything from cotton in Uzbekistan to tea in India to shrimp in Southeast Asia. Experts estimate that some $150 billion dollars (€128 billion) in profits are generated by enslaving people every year. Countries with a poor record of tolerating slave labor range from North Korea to Qatar.

Western consumers also reinforce modern slavery in their demand for goods: the cheaper, the better.

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"Slave labor is concealed behind our tomatoes and cucumbers, but also behind our mobile phones…and even bio-fuels," Anger says. "People shouldn't necessarily feel guilty. We just say: ask questions, pay a bit more attention to prices and inquire about alternatives to certain products."

Activists say that behind unbelievable bargains someone must be suffering

What you can do

Gemeinsam für Afrika and other groups calculate that an average of 60 slave laborers work for the typical Western consumer. Activists call for people to be more circumspect about what they buy, particularly if they search for bargains.

"At the very least prices are an indication for consumers," Anger says. "When something is unbelievably cheap, it's easy to imagine that it's been produced extremely cheaply and that at very end of the chain, there's someone who received even less. If I buy a kilo of meat for four euros it can't have been produced fairly."

Anger says that even in Germany, with its relatively robust labor and human rights guarantees, workers are forced to labor under slave-like conditions. The activist names the meat-production industry as one example.

She's calling on German authorities to do more to enforce existing anti-labor-exploitation laws and to fight abuse by cracking down on the use of sub-contractors as a way of flouting legal protections for workers.