A major Arlington County water main ruptured early yesterday, disrupting service to more than a third of the county's residents and threatening countywide emergency restrictions for much of the summer, when water demand is highest.

The restrictions, which county officials say could be imposed for as long as two months, would prohibit virtually all outdoor water use, including watering lawns, washing automobiles and refilling swimming pools that now are less than three-quarters full. Restaurants would be ordered to serve water only upon the request of customers.

Violators could be subject to misdemeanor charges, punishable by up to one year in jail and a $1,000 fine, officials said.

The County Board is expected to approve the emergency water restrictions at its regular meeting today, according to board member John G. Milliken. County water department officials said Arlington would have enough water for indoor home and commercial use, including air conditioning units, if the county's 155,000 residents adhere to the restrictions.

The break, which disrupted water service to about 60,000 residents for five hours, occurred at about 5 a.m. yesterday in a 48-inch water pipe in a 35-foot-deep section of the Potomac River about 100 feet below Chain Bridge.

Officials said they did not know the cause of the break, which occurred in the largest of the four pipes that are used interchangeably to bring water from the Dalecarlia Water Treatment Plant in the District into Arlington. They said the most likely cause of the break was a ruptured valve or a slow leak that finally caused a break.

"Everything that has happened so far is the worst that could happen and it is continuing in that direction," said Henry Hulme, director of the county Public Works Department. "I've never seen a break like this. In terms of supply, this couldn't happen at a worse time."

Hulme said water use in the summer is 15 percent greater than in any other season. This summer has been unusually hot and dry, taxing water supplies even more than usual, officials said.

Repairs to the pipe could take several months and cost millions of dollars, officials said, adding that they do not know the extent of the damage. The main, made of reinforced concrete, runs under six to eight feet of bedrock and is covered by a concrete shield, officials said.

Water was diverted to the three 20-inch pipes, which were operating at full capacity yesterday and supplying about 85 percent of the county's water needs, officials said. Pressure in the eastern part of the county is expected to remain weak until Sunday, officials said.

Much of the water supply to the eastern section was cut off from about 5 a.m. to 10 a.m. yesterday, officials said. Residents throughout the area awoke to find either no water or streams of brown water from faucets. Air conditioners that rely on water-cooled systems were shut down in dozens of buildings throughout the county.

"This morning was an emergency situation," said Stevan Hynson, spokesman for the Arlington Fire Department. "If we'd had a major fire at 5 or 5:30 a.m., it would have been critical."

Hynson said the water shortage forced the fire department to borrow three huge water-tanker trucks from National Airport and one from Fairfax County. He said two of the trucks were used to pump water into cooling tanks at C&P Telephone Co. facilities in the county. Those tanks must be properly filled for emergency equipment to work, he said.

Michael Kossey, who lives on North Edgewood Street, said he realized he had no water at 6:15 a.m. Kossey said he was forced to shave with bottled water, adding, "It was very unpleasant."

The break also affected water pressure in Northwest Washington when the rupture drained the water flow. Pressure in the District returned to normal after 7:30 a.m. when the flow of water through the pipe was cut off, officials said. There were no stoppages in the District, and the city will not be subject to restrictions, officials said.

The rupture also has forced Arlington to stop piping 1.5 million gallons of water a day into Fairfax County's reservoir to raise its water levels. Fairfax County is not expected to suffer any shortages, however, officials said.

Arlington County normally gets its water from the Army Corps of Engineers facility in the District. The plant treats about 150 million gallons of water a day and also serves parts of Washington and Fairfax. Arlington receives about 25 million to 30 million gallons daily.

Officials said the 48-inch pipe, which was installed in the late 1960s and could pump nearly 25 million gallons of water daily, had a life expectancy of about 50 years.