Channel ferry services, however, were returning to normal as the fishermen lifted their blockade of ports after the announcement of a pounds 16m aid package by the government.
Rennes had started to recover its calm as midnight approached on Friday, in the wake of violence which left 77 people injured. Suddenly 50ft flames erupted from the Parlement de Bretagne, a 17th century building that is the centre-piece of the city and a symbol for the Breton people.
The building's roof, containing wooden beams fashioned by the ship builders of St Malo, collapsed in the blaze. The painted wooden ceiling of the grand chamber, also destroyed, was considered to be a fine example of 17th-century art; the only one like it in France is at Versailles. Hundreds of documents of the regional appeals court, housed in the building, went up in flames.
The local prosecutor's office opened an investigation to determine whether the fire was started deliberately. One theory is that it may have been set off by a distress flare fired during the fishermen's protest.
Fire fighters, watched by a crowd of about 1,000, many of whom were in tears, took three hours to bring the flames under control. Two fire fighters were slightly hurt.
Edmond Herve, the Socialist mayor of Rennes and a former health minister, who was beaten up during Friday's disturbances, spoke of his 'great sadness before a symbol of Brittany in flames'. Mr Pierre Mehaignerie, the Justice Minister and a Breton, said he felt 'amazement, sadness and anger. The history of Brittany has gone up in smoke'.
The fire, in a building which served as the Breton parliament until the Revolution of 1789, was a spectacular climax to the violent protests that have broken out across France.
Yesterday fishermen, complaining about low prices and cheap foreign competition, ransacked two frozen food stores near La Rochelle. At St Brieuc they broke into a depot and destroyed 20 tons of frozen fish. Similar incidents were reported elsewhere.
Cordons of fishing boats stopped ferry movements overnight at Calais, Dunkirk, Boulogne, Le Havre and Carenton, stranding hundreds of vehicles and thousands of passengers on both sides of the Channel. The blockade was lifted at noon, after hours of negotiations, as fishermen's leaders met to consider government offers of aid.
'We won't stop until the borders have been sealed to imports, prices raised and our future secured,' one fisherman told France Info radio.
The trouble started early in the week in fishing ports, and spread when 1,200 protesters invaded food markets at Rungis south of Paris on Thursday, before taking off to rampage through the Channel ports. It was then that the disturbances began to take on aspects of a peasant revolt.
Despite two packages of aid announced by the government, one in haste on Friday afternoon as Mr Balladur was visiting Rennes, the violence continued, presenting the Prime Minister with his most serious crisis since taking office in March.
His low-key style with its appeal to reason had little effect on angry fishermen, whose small family concerns cannot compete with the modern methods of Russian and East European factory ships.
The Parlement de Bretagne, destroyed in yesterday's fire, has in recent months been the headquarters for an investigation conducted by Renaud Van Ruymbeke, one of France's most famous examining magistrates, into illegal funding of political parties. It is not clear whether Mr Van Ruymbeke's files were among those that were destroyed.
Last night Mr Balladur promised the people of Rennes that the government would pay for the restoration of the ruined parliament building.
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