(adds Bulgarian reaction paragraphs 2, 13-16)

By Vassilis Triandafyllou CORINTH, Greece, Jan 26 (Reuters) - Farmers demanding compensation for low agricultural prices blocked Greek highways for an eighth day on Monday, shutting border crossings with Bulgaria and cutting the main roads from Athens to major cities.

Bulgaria asked the European Commission to force Greece to reopen the border and compensate truckers, after tonnes of fruit and meat rotted and drivers faced penalties for late deliveries.

Greek officials held urgent talks with farming union leaders on Monday, but Agriculture Minister Sotiris Hatzigakis said the government would not increase a 500 million euro aid offer.

"The state is determined to stick to this policy," Hatzigakis said.

Farmers complain that high fuel and fertiliser prices last year drove up their costs but a global economic slowdown has led to rock-bottom prices, leaving many of them facing penury.

Thousands of farmers demanding tax rebates and subsidies kept the three border crossings into Bulgaria shut on Monday and barred the highway between Athens and Greece's second city of Thessaloniki, in the north.

Greek farmers have no particular issue with Bulgarian imports and in recent days have also shut the borders with Macedonia, Turkey and Albania.

For the first time, scores of tractors blocked the highway over the narrow isthmus linking the Peloponnese peninsula to central Greece. Only one other road joins the Peloponnese, home to a tenth of Greece's 11 million population, to the mainland.

"We don't intend to stop unless the government satisfies our demands," said Giorgos Goniotakis, head of the Greek Federation of Farmers Unions. "We asked the government to supplement our income, tackle cartels and fraud, but it hasn't taken steps."

Farmers' leaders say the proposed 500 million euro package remains too vague and they want details of exact subsidies on specific products, such as cotton and olive oil.

Business groups have urged the government to resolve the dispute as quickly as possible, saying transport chaos was worsening the impact of a global slowdown.

The suddenness of the downturn has angered many Greeks, after years of growth sowed hopes of prosperity. Fears of rising unemployment fuelled the worst riots in decades in December.

In a letter to the EU's executive, the Bulgarian Transport Ministry demanded Athens open a corridor for Bulgarian trucks.

"Our request to the European Commission is to alert Greek authorities that such a situation, without any reaction from their side, in practice affects the main freedoms of EU citizens," the ministry said in a statement.

The ministry also sent Brussels a statement from the Association of Bulgarian Transport Companies, seeking compensation for losses, estimated at over 10 million euros.

"Is it our fault someone has internal problems?" said Mirolub Stolarski, its chairman. "If Greece refuses, we'll go to court". (Additional reporting by Renee Maltezou in Athens and Tsvetelia Ilieva in Sofia, writing by Daniel Flynn; editing by Alison Williams)

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