BOSTON -- A massive rupture in a critical water main that supplies water to millions of residents in the Boston metro-area prompted Massachusetts Gov. Deval Patrick to issue a state of emergency Saturday, the governor urging residents in 30 communities to thoroughly boil water before drinking it.

The water is not suitable for drinking," said Patrick, but will be safe for bathing, flushing and fire protection.

The water main break, which was brought under control about 6:45 p.m. when a valve was shut down, affected service to 2 million customers, or 700,000 households. With 64 million gallons of water lost before the flow was capped, officials had to use untreated water from backup reservoirs to continue providing water to the region for firefighting and sewage.

"Failure to boil water rapidly for at least one minute could cause serious illness," Patrick said, explaining that the Massachusetts Water Resources Authority's backup water supplies were coming directly out of reservoirs. That meant the water did not meet strict federal standards for drinking water supplies.

"What we're having to do is use reservoirs with untreated water," Patrick said.

The MWRA said the leak was in a relatively new, 10-foot steel water pipe buried 12 feet underground in Weston, a community west of Boston. The main connects the Quabbin Reservoir to MWRA-supplied communities surrounding Boston. At one point, 8 million gallons of water an hour was pouring into the Charles River and 38 communities were affected. Many came off the MWRA list of affected communities, however, when they switched to local water sources.

MWRA chief Frederick Laskey called the break "highly unusual and catastrophic."

"It's a very substantial break in a very large pipe," Lasky said, advising residents in the affected communities to conserve water and avoid unnecessary activities such as car washing and lawn watering.

Lasky said the custom-made water main was only six or seven years old and it was not immediately clear what caused it to rupture sometime between 10 a.m. and 11 a.m. on Saturday.

"It could have been a design flaw, it could have been a construction flaw, it could have been that the product was faulty, it could have been something in our system," Laskey said.

He said officials would likely not have an estimate on how or when it will be possible to completely repair the pipe until Sunday morning at the earliest.

"It's not as if there's (a pipe) that we can go pick up and replace it," Lasky said. He said he hoped the main could be repaired in "days not weeks."

The area where the leak occurred is near Recreation Road at the juncture of Route 128 and the Massachusetts Turnpike and affected communities east of Weston. (See list.)

The MWRA activated its emergency water supplies at the Sudbury Aqueduct, Chestnut Hill Reservoir and Spot Pond Reservoir.

Boston Mayor Thomas Menino said he also issued a state emergency in the city, and officials urged all residents in MWRA communities to check on elderly and vulnerable neighbors to ensure that they are complying with the boil water order.

Menino said Boston residents were being alerted about the boil water order through the city's reverse 911 system and by the use of bullhorns and fliers that were being distributed. Restaurants were also being advised to take precautions.

Boston residents with concerns were told to call the Mayor’s 24-Hour Hotline at (617) 635-4500 which planned to have extra staff on hand to help assist with questions. Information was also available on www.cityofboston.gov.

Patrick said the state would be asking stores to increase supplies of bottled water and, if necessary, would utilize the National Guard to provide water to communities if necessary.

The Massachusetts Society for the Prevention of Cruelty to Animials or MSPSCA-Angell reminded pet owners to follow the same protocol for their pet’s water supply as they would for themselves.

Pet owners who believe that their pet has fallen ill due to drinking contaminated water were urged to bring their pet to an emergency veterinary clinic.

Updates can be checked here and on the MWRA website. The Emergency Operations Center is open. The phone number is (617) 305-5970.