Protesters attend a demonstration against the transatlantic trade deal (TTIP) on the eve of U.S. president's visit to Hanover, northern Germany, April 23, 2016. JOHN MACDOUGALL/AFP/Getty Images

U.S. President Barack Obama praised German Chancellor Angela Merkel ahead of his visit to the European country Sunday, telling German newspaper, Bild, that he was proud to call her a friend. This would mark the last destination of the U.S. leader's six-day foreign trip, through which he has attempted to boost America's alliances with U.K. and Gulf leaders.

Obama, who is set to meet Merkel shortly after he reaches Hanover, Germany, is expected to discuss about topics including global economy, terrorism, transatlantic security and a free trade pact between the U.S. and Europe, according to a White House press briefing.

On Sunday, Obama is set to speak at the Hannover Messe, the world’s largest industrial trade fair, and drum up support for the Transatlantic Trade and Investment Partnership (TTIP), a U.S.-Europe free trade agreement that supporters say could boost each economy by some $100 billion, according to Reuters.

However, tens and thousands of people gathered in Hanover’s Opera Square on Saturday to protest the proposed deal as they fear a lowering of environmental and social standards in Europe. TTIP opponents were seen wearing T-shirts that read "Stop TTIP" and carrying "Give TTIP no chance" slogans.

“We are here to make it clear that we have concerns and that is true not only for the German and European population. Americans are worried too,” German Green Party co-leader Simone Peter told euronews. Local police said 35,000 people took part in Saturday’s demonstration in Hanover, while organizers said more than double that number had attended, reports said.

Both leaders announced Saturday that they support free trade pacts between the U.S. and Europe as it would fuel growth on both sides of the Atlantic.

"The Trans-Atlantic Trade and Investment Partnership is one of the best ways to promote growth and create jobs," Obama told Bild on Saturday. Merkel said that wrapping up a deal would be a "win-win situation,” in her weekly podcast.

Last year, the United States overtook France to become Germany's most important trading partner, according to official data released in March.