New Delhi, Dec. 6 -- Riot police fired tear gas Friday at farmers attempting to block the main access road to the international airport in Thessaloniki, Greece's second-largest city, as protests escalated over delays in the payment of European Union-backed agricultural subsidies.

Irate farmers have deployed thousands of tractors and other agricultural vehicles at border crossings and key points along highways across the country, periodically stopping traffic and threatening to completely blockade the roads, as well as airports and ports.

Police are enforcing traffic diversions in several parts of northern and central Greece to skirt the blockades, while farmer roadblocks at the country's northern borders with Bulgaria, Turkey and North ...