Ten years after the sinking of the Prestige oil tanker off Spain, the ship's Greek captain and three others went on trial on Tuesday over the worst oil spill in the country's history.

The first day of the trial, held at an exhibition centre in the northern port city of A Coruna, was dominated by procedural questions, with the accused only expected to take the stand in November.

Apostolos Mangouras, 78, the Prestige's captain, is charged along with two other officers and a Spanish official over the oil spill, which polluted thousands of kilometres (miles) of coastline in Spain, Portugal and France.

Prosecutors are demanding 12 years in jail for Mangouras, who is charged with harming the environment along with Greek chief engineer Nikolaos Argyropoulos and first mate Irineo Maloto, a Filipino whose whereabouts are unknown.

The fourth defendant is Jose Luis Lopez-Sors, head of the Spanish merchant navy at the time, who ordered the ship out to sea when it was losing fuel.

But environmental groups complain that key people responsible for the disaster were missing from the trial and warned that the lessons from the oil spill -- one of the worst in history -- had not been learnt.

"There are many people who should be in the dock as well who are not there," said the coordinator of Greenpeace Spain campaigns, Maria Jose Caballero.

Among those who should also be held accountable is Spanish Prime Minister Mariano Rajoy, who was deputy prime minister and government spokesman at the time of the accident, she added.

"We feel that the government's management of the accident bordered on negligence," said Caballero.

The conservative Popular Party government in power at the time ordered the Prestige out to sea away from the Spanish coast instead of following an emergency contingency plan prepared by experts that called for it to be brought to port where the leaking oil could be confined.

Rajoy initially downplayed the gravity of the accident, repeatedly describing the black spots that appeared in the sea where the tanker went down as "small threads of clay".

Spanish non-government group Environmentalists in Action said charges should be brought against ABS, the marine classification company that certified the ageing Prestige as seaworthy, and complained that such single-hull tankers are still being used to transport petrol.

The Prestige leaked 50,000 tonnes of fuel into the Atlantic after it sank off northern Spain. It took on water in a storm on November 13, 2002, and drifted for six days before breaking up and sinking.

Over the weeks that followed 300,000 volunteers from Spain and the rest of Europe joined local people in scraping the oil from the rocks and beaches, armed with little more than buckets and their bare hands.

-- Increased compensation demand --

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Prosecutors had originally demanded 2.2 billion euros ($2.9 billion) in damages but on Tuesday they increased the amount to just over four billion euros.

The higher amount was based on a study carried out by the University of Santiago de Compostela, which put the total cost of the environmental damage caused by the oil slick at 4.1 billion euros, most of it for the Spanish state.

According to Greenpeace, the Prestige was built in 1976. It belonged to a Liberian-based shipping company and sailed under a Bahamas flag of convenience.

Mangouras, wearing a dark blue suit and tie, attended the opening session of the trial along with Argyropoulos and Lopez-Sors.

Outside the court about 300 people braved rainy weather to demonstrate against the government's handling of the oil spill and demand that more people be held accountable for the disaster.

They chanted "No impunity" and "Oil spills, never again". Several held up signs bearing a picture of Rajoy above the caption "100 percent liar".

After another day of procedural matters on Wednesday, the court is due to hear from the defendants from November 13, the 10th anniversary of the disaster.

The trial is due to last until May and hear testimony from 133 witnesses and 100 experts.