Earlier, farmers ended a blockade at the airport of Iraklio, on the island of Crete. The airport, Greece's third-busiest, closed Tuesday when farmers drove tractors inside the flight area and set tires on fire.
A meeting late Wednesday between farming association leaders and Agriculture Minister Sotiris Hatzigakis failed to produce a breakthrough.
"These problems cannot be solved immediately," Hatzigakis said after the talks.
Government spokesman Evangelos Antonaros said the government was "studying" farmers' demands, but was constrained by the effects of the global economic crisis.
The farmers are demanding a list of measures including higher subsidy and pension payments and lower fuel taxes.
"We reached a point where many farmers can no longer make a living," protest organizer Thanassis Kokkinoulis said in the central city of Larissa. "The government must provide these people with assistance."
Elsewhere, farmers used thousands of tractors to stage sporadic blockades along highways including the main one linking Athens with Greece's second-largest city, Thessaloniki. They also maintained closures at two border crossings with Bulgaria.
Police said farmers and stranded truck drivers clashed briefly overnight at one border post. Farmers also set fire to trash bins. No arrests or injuries were reported.