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HANNOVER, Germany — About 35,000 protesters marched Saturday against the proposed Trans-Atlantic Trade and Investment Partnership (TTIP), a day ahead of Barack Obama's visit, police said.

The president will use the annual CEBIT international trade fair in Hannover, in northern Germany, to promote the deal, which would create the world's largest free trade area, with about 800 million people.

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"A convoy with 30 tractors already started this morning and will be met by a large number of protesters in the city center," police spokeswoman Martina Stern told NBC News.

Protesters demonstrate against the Transatlantic Trade and Investment Partnership (TTIP) free trade agreement ahead of U.S. President Barack Obama's visit in Hannover, Germany. Kai Pfaffenbach / Reuters

The protest was organized by the so called "anti-free-trade alliance" made up of numerous associations, including trade unions, environmental activists and church groups, who fear the TTIP could threaten Germany's environmental and legal standards.

Analysts say that protesters in Germany rarely express worries about negative economic impacts of transatlantic trade.

Instead, local criticism "has often focused on an increase in what is perceived as undue corporate influence, a feared lack of democratic control, as well as worries about threats to existing labor, environmental, and consumer protection standards in TTIP," Peter Sparding from the German Marshall Fund (GMF) wrote in an essay this week.

The growing skepticism was also reflected Friday in a survey by Germany's Bertelsmann Foundation, which found that only 1 in 5 Germans are in favor of TTIP.

The United States has seen strong public support for free trade, but there is particular uncertainty about TTIP. According to the Bertelsmann survey, "18 percent of US residents oppose the TTIP free trade agreement, while only 15 percent support it."

Slogan 'Yes, we can Stop TTIP': more than 25k people protest EU-US trade pact ahead of #Obama visit, police tell @NBCNews, #ttipdemo #TTIP — Andy Eckardt (@ameckardt) April 23, 2016

The "widespread and shared concern" regarding the current trade negotiations could be "a perceived lack of transparency," as Sparding wrote.

In a speech marking the 500th anniversary of Germany's beer purity law, Chancellor Angela Merkel argued that medium-sized businesses, for example in the beer industry, could well benefit from a transatlantic trade partnership.

Quoting German 16th-century reformer Martin Luther, Merkel said: "He who does not have a beer, has nothing to drink."