Facing the largest boil-water order in the region’s history, Portlanders emptied stores’ supplies of bottled water, chucked potentially contaminated food and braced for a complicated Memorial Day weekend.

About 670,000 people in the Portland, Oregon, area were told after lunch on Friday to boil water used for drinking, cooking and tooth-brushing. Three routine tests this week showed the water system was contaminated by E. coli bacteria, usually associated with animal or human faecal matter.

The finding suggested the water supply could include other harmful bacteria, although health officials said the worst condition someone likely would suffer is diarrhoea.

Within the city, nearly every grocery store - from mom-and-pop shops to Wal-Mart - reported that they were out of water by the evening. At Grocery Outlet in North Portland, staffers said it took about half an hour to run out of the large jugs of water. People then started buying sparkling water, juice and ice.

“It’s a weird combination of cabin fever, ‘Road Warrior’ and doomsday scenario preparation,” a store employee said by phone. A manager said she tried to order more water supplies for the weekend, but “they aren’t coming,” she said.

“It’s Memorial Day weekend, and no one’s answering phones or anything,” she said.

At least one grocery chain, Fred Meyer, was expecting fresh shipments of bottled water late Friday.

Stores and restaurants in neighbouring towns were on edge - not sure whether the water restriction would bring big crowds and counteract the usual holiday slowdown.

At the Old Spaghetti Factory in Clackamas, a manager said they had plenty of clean water.

Authorities said the boil order would be lifted as soon as the water tested safe, which could be as early as yesterday morning.

In the meantime, hotels warned their guests not to drink from the taps. The Jupiter Hotel, near the Lloyd District shopping hub, pulled water coolers into the lobby and posted warning signs anywhere a guest could access tap water.

Restaurants, cafes and the city’s famous craft brewers responded in different ways. Several closed for the day, including Stumptown Coffee. On Twitter, the chain said, “It’s an extra safety precaution related to rinsing milk pitchers, ice, dish washing, etc. Sorry about the inconvenience!”