French farmers blocked roads from Spain and Germany on Sunday to stop foreign products entering the country, the latest protest against a fall in food prices that has brought them to the brink of bankruptcy.

Farmers in the northeastern Alsace region used tractors to obstruct six routes from Germany in a bid to stop trucks crossing the Rhine carrying agricultural goods, in a blockage that is expected to last until at least Monday afternoon.

"We let the cars and everything that comes from France pass," Franck Sander, president of the local branch of the powerful FDSEA union, told AFP, adding that more than a thousand agricultural workers were taking part in the roadblocks.

A dozen trucks have been forced to turn back at the border since the blockage started at around 10pm (2000 GMT) on Sunday night, according to a union official.

Meanwhile, around 100 farmers ransacked dozens of trucks from Spain on a highway in the southwestern Haute-Garonne region, threatening to unload any meat or fruit destined for the French market.

They used some 10 tractors to block the A645 motorway not far from the Spanish border, causing traffic jams that stretched up to four kilometres (2.5 miles), Guillaume Darrouy, secretary general of the Young Farmers of Haute-Garonne, told AFP.

The action comes after a week that has seen farmers block cities, roads and tourist sites across France in protest at falling food prices, which they blame on foreign competition, as well as supermarkets and distributors.

Farmers have dumped manure in cities, blocked access roads and motorways and hindered tourists from reaching Mont St-Michel in northern France, one of the country's most visited sites.

Fearful of France's powerful agricultural lobby, the government on Wednesday unveiled an emergency package worth 600 million euros ($660 million) in tax relief and loan guarantees, but the aid has done little to stop the unrest.

"The measures announced by the government ... none of them deal with the distortion of competition" with farmers from other countries, said Sander, saying French farmers face higher labour costs and quality standards.

A combination of factors, including changing dietary habits, slowing Chinese demand and a Russian embargo on Western products over Ukraine, has pushed down prices for staples like beef, pork and milk.

Paris has estimated that around 10 percent of farms in France -- approximately 22,000 operations -- are on the brink of bankruptcy with a combined debt of 1.0 billion euros.