State officials were optimistic about repairing by Sunday night a pipe that broke Saturday, saying roughly 2 million residents in metropolitan Boston would likely have to wait days before counting on uncontaminated water flowing from their taps.

"It does look like we're going to be able to get through this sooner than we thought yesterday - days, not weeks," Gov. Deval Patrick said Sunday afternoon.

Patrick urged residents to rely on boiled water as "the first solution."

Massachusetts Water Resources Authority (MWRA) officials said Saturday night they had been able to stop the leak in the pipe that connects two aqueducts in Weston. By Sunday, they had excavated the pipe and begun repairs, which Patrick said were "well along" Sunday morning, with completion expected as early as Sunday night.

Once the pipe is repaired and subjected to pressure testing to test its integrity, the MWRA and Department of Environmental Protection will conduct water quality tests to ensure safety.

"The goal is to have it repaired tonight," said MWRA Commissioner Fred Laskey, adding, "There could be some collateral damage that we can't see, because there was so much water moving through there."

Describing himself as "a little punch drunk" about 30 hours after the pipe's breach Saturday morning, Laskey said the state had withstood a large increase in the use of water driven by Sunday's weather, when temperatures topped 75 degrees. Once the pipe is safely repaired, Laskey said, the system can stop drawing water from the Chestnut Hill Emergency Reservoir, which is heavily chlorinated but still unsafe for ingestion.

After two days of safe-water tests, the system can resume functioning, Laskey said. He said tests are conducted in the morning and the evening, but declined to put a date on a return to full capacity.

"We're very, very reluctant to set any expectations … because we can't predict the unpredictable," Laskey said, adding, "For the people in the water industry, it is everyone's worst nightmare … it is something that you read about in the history books."

Patrick said he had not aggregated expected costs, which are likely to include repairs, overtime and emergency services.

Schools and businesses were expected to open Monday, with precautions planned like prepackaged meals. At a press conference after he convened his Cabinet Sunday afternoon at the state's emergency management bunker in Framingham, Boston Mayor Thomas Menino repeatedly mentioned that Boston restaurants would be open for business.

Laskey bemoaned the timing of the breach, about a year after the MWRA launched a $50 million contract to fix the Hultman Aqueduct, intended to serve as a backup in disaster scenarios. Officials said the repairs are roughly 25 percent complete.

Laskey said, "Right at the worst time, this happened … at the most vulnerable time."

Water pressure, vital to fire hydrants, remained stable Sunday afternoon.

Water into the system was coming Sunday from the Carroll Treatment facility, supplemented by the Chestnut Hill Emergency Reservoir.

Speaker Robert DeLeo plans fact-finding hearings on the incident, his spokesman said Sunday.

Patrick directed consumer protection officials to monitor bottled water prices in the region, including store inspections.

"We expect businesses to operate responsibly and to treat customers in an appropriately sensitive manner," Patrick said in a statement. "There is never an excuse for taking advantage of consumers, especially not during times like this. I have asked our Division of Standards to closely monitor the situation so that we are protecting consumers."

Laskey on Saturday called the pipe breach in Weston "unprecedented" and "highly unusual and catastrophic in its nature."

The MWRA activated emergency water supplies Saturday, but said that water was not drinkable, and should be used instead for bathing, flushing and other household use. To safely boil for consumption, teeth-brushing and hand-washing and shaving, officials said, the water should come to a rolling boil for at least a minute.

Health and Human Services Secretary JudyAnn Bigby said the state was trying to preserve its bottled water stock for people without reasonable alternatives.

The 10-foot-diameter steel pipe is relatively new, connecting the Boston tunnel with the Metrowest tunnel, which was built to augment the Hultman Tunnel, MWRA spokeswoman Ria Convery said.

Officials said over 700,000 households in 30 cities and towns were affected. The municipalities Convery listed were: Arlington, Belmont, Boston, Brookline, Canton, Chelsea, Everett, Lexington, Lynnfield Water District, Malden, Marblehead, Medford, Melrose, Milton, Nahant, Newton, Norwood, Quincy, Reading, Revere, Saugus, Somerville, Stoneham, Stoughton, Swampscott, Wakefield, Waltham, Watertown, Wilmington, Winchester and Winthrop.