About 500 tractors blocked the A1, which goes to northern France and Belgium, overnight.
Others stopped tourists reaching Normandy's famous Mont St Michel, closed at least four bridges and hindered traffic on roads and motorways.
Most of the action has so far been in Normandy but it spread to other regions on Wednesday morning.
Blockades were reported in the Vendée and Poitiers, south-west of Paris, Amiens and Abbeville in Picardy and the Rhône-Alpes region, south of the capital.
A meeting with Agriculture Minister Stéphane Le Foll failed to calm the protesters.
"The farmers will not let up for the simple reason they are fighting for their lives," Jean-Pierre Fleury, the leader of a beef farmers' union, said.
The government decided to provide 600 million euros to help farmers in difficulty, Le Foll announced after the cabinet discussed a 24-point emergency plan on Wednesday morning.
Farmers' debts should be restructured, he said, and social security contributions will be cut.
Some 500 million euros will be placed in a fund to provide credit and 10 million euros will be invested in promoting meat exports.
A government-commissioned mediators' report has found that a June agreement to raise prices has almost been reached its targets for pork - which now sells at about 1.40 euros per kilo - but not for beef, which has risen 10 centimes a kilo rather than the planned 20 centimes.
Farmers' union representatives said that there was uneven commitment to the plan by abattoirs, food processors, restaurants and supermarkets, whom they accuse of pushing down prices in the past.
They also say they face unfair competition from Germany and Spain, where casual, often immigrant, labour is as much as nine euros cheaper than in France.
Pork and milk prices have suffered from the European Union's embargo on Russia over the Ukraine conflict and the end of the EU's milk quotas.
There are about 900,000 jobs directly or indirectly involved in the sector and about 22,000 farms are reported to be on the brink of bankruptcy.