By 11 this morning, the shelf displaying bottled water was empty at Sainsbury's supermarket in Chiswick, West London, and the cartons of long-life milk were going fast.

"Maybe I feel a little foolish," said Margaret Jackson, a mother of two children who had a shopping cart full of water, dried pasta and canned food. "But I read the newspapers, and I watch television, and I listen to our leaders telling us to be prepared. So that's what I'm doing."

Stoicism in the face of danger is a great British tradition. Unlike in Washington, people here have not been lining up in vast numbers to strip hardware stores of plastic sheeting and duct tape, nor have they planned escape routes from central London in case of a chemical attack.

But as Britain joins the United States in waging war in Iraq, a small measure of alarm is starting to spread, aided by a stealth-like government campaign to quietly alert Britons to the threat of terrorism.

Two days ago, the Home Office, which is in charge of internal security, placed unannounced on its Web site a series of "sensible precautions." It suggested that residents maintain a supply of bottled water, canned food, blankets and batteries, along with a flashlight and battery-powered radio.

A department spokesman insisted the advice was relevant for any emergency and was not meant to be a terrorism warning. "We're not advising people to start panic-buying," he said.

Home Secretary David Blunkett told the House of Commons that the government was "taking every feasible precautionary measure" to protect the public. But he also warned: "The terrorist threat remains real and is serious."

Some Londoners have taken the warning to heart. The Home Office guidelines state that "there is currently no information that would lead us to advise you to obtain protective clothing, including gas masks." Still, Edward Klinger, chief executive of Ozonelink, a London-based security supply firm, said his company has received more than 60,000 inquiries in recent weeks about purchasing gas masks and protective clothing.

"The numbers we're seeing are incredible," Klinger said. Based on current sales activity, he said he expects over the next week to sell twice as many of the company's most popular item, a gas mask that retails for about $160, as he sold during the past three months.

"People are getting anxious, and you can lay a lot of the blame on the government," Klinger said. Referring to the prime minister and the London mayor, he said that "Tony Blair and Ken Livingstone have said many times that an attack is inevitable. People are starting to put the puzzle together and say, 'Hold on, I need something for protection.' "

Some segments of the London community have quietly been on alert since the Sept. 11, 2001, terrorist attacks in the United States. Michael Whine of the Community Security Trust, a nonprofit organization set up to protect Jewish institutions in Britain, said his group long ago posted guards and took other security precautions at every synagogue, school and other Jewish-related site in London.

"You don't necessarily see the security; it's not as overt as in the States," he said. "But it's there."

"Unlike America, we've been through this before. We had the IRA for more than 20 years," he said, referring to the Irish Republican Army, which waged violence in London as part of its campaign to merge the British province of Northern Ireland with the Republic of Ireland. "And before that we had the [World War II] Blitz. It's also part of the British character. We're more phlegmatic and less excitable. Maybe it's the lousy weather and the warm beer."

Police have quietly beefed up patrols in London, adding between 800 and 1,500 officers, according to John Stevens, Metropolitan police commissioner. Airports and train stations are areas of special focus, officials said, although there has been no repeat of the massive alert at Heathrow Airport this year that brought out tanks and troops because of concerns that a terrorist gang might seek to bring down a civilian passenger jet with a missile.

"It's not a question of if, it's a question of when" a terrorist attack will occur here, said a senior security official who insisted that he not be identified. He said police were concentrating on "picture postcard sites" where tourists gather, key economic areas such as the financial district, power plants and other energy sources, and institutions identified with the United States or Israel.

Officials complain that they get criticized both ways on security issues. When they dispatched troops to Heathrow in February, they were accused by critics of stoking public fears to win popular support for Blair's campaign against Iraq. When they play down threats, they are accused of hiding information.

At the same time, they have come under attack for not spending enough on emergency preparation. The national fire service still lacks many of the search-and-rescue devices, protective suits and decontamination tools it requested to deal with a chemical or biological attack, according to news reports.

Blair has told Parliament the government is spending hundreds of millions of British pounds on vaccines, protective clothing and new emergency procedures.

"We could spend billions of pounds doing it, we could spend tens of billions of pounds doing it, and we could still not identify where the attack was going to come from," he said. "There are no limits to the potential threats you can imagine."

Not everyone is getting the message. Zahoor Ashraf, assistant manager of Western Food & Wine on Turnham Green Terrace in west London, said an elderly woman came to the store at 7 a.m. Thursday and bought three large bottles of water, some canned fruit and a box of candles. With the Iraq war just getting underway, he expected to see many more worried customers. But it was business as usual all day long.

"That's the way most people are around here," he said. "Nothing really bothers them."