Thousands of barrels of oil from the wrecked American-owned supertanker Amoco Cadiz swept down the picturesque Brittany coast yesterday, blackening beaches and menacing ports along 40 miles of shoreline.

The gigantic oil slick, estimated to be about 10 miles wide, threatened the major port of Brest and the region's vital tourist and fishing industries and was headed southward, authorities said.

French officials said 560,000 barrels of Middle East crude oil have poured from the 990-foot tanker since it broke in half Friday after ramming a reef a mile offshore Thursday night. There are 42 gallons in a barrel of oil.

Officals fear the spillage could become France's worst ecological disaster before it is brought under control. In 1967 the tanker Torrey Canyon broke up off southwest England, 100 miles northwest of the site of the Amoco Cadiz spill, dumping 620,000 barrels of oil on the English and French coasts.

Police detained the Italian captain of the Amoco Cadiz and the West German captain of a tug that was sent to tow it before it went aground. The tanker's steering machinery had malfunctioned.

Sources said authorities will try to establish whether the Amoco Cadiz violated too close to shore before striking the reef, which would make it liable for damages. The ship, owned by the American Oil Co., a subsidiary of Standard Oil of Indiana, carries pollution insurance - mostly through Lloyds of London - but the amount was not immedately known.

The spillage was caused when three of the ship's 15 tanks ruptured. The tanker was carrying 1.5 million barrels of oil from the Persian Gulf to the port of La Havre, France. Weather was expected to remain calm for at least two days, lessening the danger of a further break-up.

All 43 crew members have been rescued from the ship by helicopter.

British pollution control experts were keeping a nervous eye on the slick. They said it poses no immediate danger to English beaches.

"It's a tragedy. This was a prosperous little town with the fishing and the tourism. Now God knows what will happen," said a woman in a cafe in Portsall, the largest of about 20 French fishing villages in the polluted area.

A harborside fish shop in Portsall stood empty. A sign read: "No fish. Oil for sale here.Direct from the well."

The Socialist mayor of Brest blamed the center-right coalition that governs France, saying nothing had been learned from previous spills.

Some analysts predicted the spill could push ecology-minded voters toward the left in the second round of voting today in France's national parliamentary elections. A combination of ecology-oriented parties gained 2.1 percent of the vote in the first round last Sunday.

An airplane with high-volume pumps and technicians from Detroit was en route to the scene to help with the cleanup. French authorities hoped to place the pumps on the tanker by helicopter and pump the remaining oil to smaller tankers before the ship breaks up.

"That's useless. They should bomb the ship immediately to set it afire. Pumping will take at least two weeks and by then I guarantee nothing will be left of the ship," sid Jules Legendre, assistant mayor of Portsall and a former tanker officer.

Bitter fishermen gathered in a harborfront cafe, ironically named "Le Recife" (The Reef), and stared at the harbor, covered in inches of black oil.

"We are ruined, ruined for at least 10 years," said one fisherman. "It's not just the fish. From Portsall we gather 90 percent of the seaweed used in France for pharmaceutical products.

"A few miles north it was only last year they got their first crop since the Torrey Canyon oil spill 11 years ago."