HANOVER (AFP) - Thousands of opponents of a proposed transatlantic trade deal poured onto German streets on Saturday (April 23) on the eve of a visit by US President Barack Obama.

Obama's trip - to open an industrial technology fair in the northern city of Hanover and hold talks with Chancellor Angela Merkel and other European leaders - was intended to lend momentum to flagging efforts to see the world's biggest trade pact finalised this year.

In an interview with German newspaper Bild ahead of the visit, Obama underscored his belief that the deal will strengthen trade and create jobs.

But the Trans-Atlantic Trade and Investment Partnership (TTIP) has run into major opposition, not least in Europe's top economy Germany, where its foes have raised the spectre of eroding ecological and labour market standards and condemned secrecy shrouding the talks.

A loose coalition of trade unions, environmentalists and consumer protection groups gathered a crowd of about 16,000 in front of Hanover's opera house ahead of a march through the city centre expected to draw around 50,000.

Amid a heavy police presence, one banner reading "Don't give TTIP a chance" featured the image of a bull tagged "privatisation" and a cow branded "democracy".

A mock coffin lay on the ground emblazoned with the words "Democracy killed by money" and "Here lies democracy".

"We are not demonstrating against Obama but against TTIP," said the head of one campaign group, Campact, Christoph Bautz.

"TTIP is deeply un-American and anti-European because it endangers our shared value: democracy." A similar protest in October in Berlin drew up to 250,000 people, according to organisers, signalling an uphill battle for the deal's passage.

In a video podcast, Merkel insisted on Saturday (April 23) that TTIP would not ride roughshod over citizens' rights or interests.

"We don't want people to have the impression that something is being hushed up here, or that norms are being undermined. The opposite is true," she said.

In what she called a "win-win situation", Europe and the United States had the opportunity to agree on environmental and consumer protection principles that, due to the massive size of the market, "could set global standards".

After talks with Obama on Friday (April 22), British Prime Minister David Cameron also insisted TTIP "would add billions to our economies and set the standards for the rest of the world to follow".

However, US Trade Representative Michael Froman told the German business daily Handelsblatt on Friday (April 22) that if the negotiators fall short, "there will be real doubts about whether we will ever get this agreement through".

The Hanover meeting comes just before a 13th round of TTIP negotiations starts on Monday (April 25) in New York.

But scepticism in the face of those arguments is growing in Germany, and Economy Minister Sigmar Gabriel admitted this week: "It is possible that TTIP will fail." Just 17 per cent of Germans say they support TTIP, according to a Bertelsmann Foundation poll of more than 3,000 people published on Thursday (April 21), in free fall from the 55 percent registered two years ago.

During the same period, firm opposition to the pact rose to 33 per cent from 25 per cent.

The picture in the United States is hardly more promising.

The "Yes" camp has shrunk to 15 per cent from 53 per cent, while nearly half - 46 per cent - say they feel too ill-informed to have an opinion.

US Agriculture Secretary Tom Vilsack, the longest serving member of Obama's cabinet, said on a visit to Germany this month that the time had come to rally around the accord, also in the interest of global food security.

With a protectionist streak running through the presidential race from both major parties, Obama's successor at the White House would likely be a less ardent backer of free trade.

Given the lack of political capital available to a "lame duck" president, US and European analysts said, the White House was more likely to aggressively pursue ratification of one of Obama's signal achievements, the Trans-Pacific Partnership trade deal (TPP) with Asia, than to struggle to complete negotiations on TTIP.