* Public support crucial to achieve world's biggest trade deal
* Upswing of opposition over past year, fears of 'Franken-foods'
By Robin Emmott
BRUSSELS, May 15 (Reuters) - The United States and the European Union sought on Thursday to counter rising opposition to transatlantic trade talks as hundreds of activists protested against the plans that they say would only benefit big companies fixated on maximising profits.
A year into talks to create the world's largest free-trade pact, European farmers, unions and nationalist politicians are trying to sway an uncertain public against the plans, despite studies showing an accord could increase economic growth.
Banging drums, around 500 people tried to form a human chain around a business conference in Brussels, hours before the EU's trade chief and the U.S. ambassador to the European Union spoke to defend the project. Police said they briefly detained 240 people.
From Germany to Greece there is growing hostility to the proposed deal - known as the Transatlantic Trade and Investment Partnership (TTIP) - which is uniting far-left and far-right parties running in European Parliament elections next week.
"The TTIP is going to damage our personal freedoms, our food safety laws and our welfare protection," said Lazaros Goulios, a Belgian trade unionist. "These are secret negotiations. Who knows what is happening?"
But European Trade Commissioner Karel De Gucht, who hopes the U.S.-EU trade deal can be agreed by the end of next year, accused activists and politicians of inventing claims, for example that the EU would open up to U.S. beef produced with growth-enhancing hormones.
"Using social media, a number of NGOs are using arguments that are sometimes complete lies," De Gucht said, speaking to business groups along with U.S. ambassador Anthony Gardner.
"This is being taken on by the leftists and the greens who see the TTIP as something to be defeated in the European elections," he said, referring to the May 22-25 vote.
'OBLIVIOUS' TO BENEFITS
The European Union and the United States already trade almost $3 billion in goods and services each day. By deepening economic ties, the pact could create a market of 800 million people where business could be done more freely.
Studies show a deal encompassing almost half the world's economy could generate $100 billion in additional economic output a year on both sides of the Atlantic - particularly significant for the euro zone as the bloc recovers from a two-year recession and near-record unemployment.
However, some groups say that is too optimistic because neither the United States nor the EU will go far enough in reducing barriers for small companies to feel the benefit.
According to a poll by the Washington-based Pew Research Centre, only 38 percent of Germans support removing all duties on goods traded between the United States and Germany, which is Europe's biggest exporter.
U.S. envoy Gardner said any increase in economic output from a trade deal should be welcome.
For instance, removing tariffs on goods would boost transatlantic trade by 17 percent and could mean up to 1 million new jobs, he said, citing an EU study.
"Have we become so rich that these numbers are not meaningful? Have we become so oblivious to the problem of unemployment that this increase is not significant?"
Public support is crucial because the European Parliament and the U.S. Congress must ratify the agreement once it is reached.
On both sides of the Atlantic, consumer and green groups worry a deal letting firms operate freely in both the EU and the United States might allow companies to bypass food safety and environmental standards.
Some Europeans are worried about what impact genetically modified crops and products - often dubbed "Frankenstein Food" - might have on health and the environment.
Trust in the United States has also been damaged by leaks about the extent of U.S. surveillance of worldwide emails and phone records, and reports that Washington bugged EU offices and German Chancellor Angela Merkel's mobile phone. (Additional reporting by James Francis; Editing by Mark Trevelyan)