French farmers for years has denounced the trade deal with the Mercosur nations of Brazil, Argentina, Bolivia, Paraguay and Uruguay, arguing the deal would hurt French farmers' livelihoods.
Thursday's protest was staged by the Rural Co-ordination union to put further pressure on France's government, which has opposed the deal.
Jose Perez, President of the Rural Co-ordination in the Lot-et-Garonne region in southwestern France, said "the goal today is to come to Paris to express our demands closer to those who have the power".
"It's a strong symbol," he told The Associated Press.
The interior ministry said about 20 tractors were in the Paris city centre, some at the Arc de Triomphe monument and others in the Eiffel Tower neighbourhood, despite a ban issued by authorities.
Convoys of tractors "bypassed and forced their way", the ministry said.
But most of the tractors were blocked further from the centre at key traffic arteries that mark Paris's limit.
Farmers' concerns about the Mercosur trade deal were combined with anger about government sanitary measures against the spread of a bovine disease, Perez stressed.
The EU this week renewed internal negotiations over a free trade agreement with five South American nations, amid speculation that a deal could be signed in Paraguay on January 12.
The deal's supporters, led by Germany, might be able to pass over the objections of France and Poland.
Fierce opposition from France derailed the deal in December.
French Agriculture Minister Annie Genevard reaffirmed France's opposition to the Mercosur deal on Wednesday, saying it threatened the production of beef, chicken, sugar, ethanol and honey, among other sectors.
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