Bombs exploded in crowded subways in a normally peaceful major European capital, turning cars packed with rush-hour commuters into twisted wreckage filled with thrashing injured.

The scene wasn't London on Thursday but Paris (search) a decade ago this month, when Algerian Islamic (search) militants carried out a series of bloody subway attacks that killed eight people and wounded about 200.

France's response — authorities welded shut trash cans and instituted a vigilance campaign — could give clues to what's in store for Londoners.

Apart from deploying bomb detection squads to scour subway stations, British officials haven't said what they might do to tighten security and thwart more attacks.

But the Paris experience with its siege, which began in July 1995 and inflicted nearly five months of terror on city dwellers, offers a scenario that could play out in London: anger, frustration and helplessness that so little can be done to stop extremists bent on spectacular attacks.

"It is absolutely impossible to prevent a determined terrorist — particularly a suicide bomber," said Alex Standish, editor of Jane's Intelligence Digest (search).

"The best scenario we can hope for is that the intelligence services are able to infiltrate terrorist groups or intercept their communications and foil their plots," he said.

French authorities reacted with a series of pragmatic measures that changed the Parisian way of life. Those included welding shut trash cans at subway stations and key monuments and museums and instituting "Operation Vigipirate," a vigilance campaign still in place today.

The campaign, similar to the United States' post-Sept. 11 terror readiness operation, is a color-coded system that puts police and other authorities on high alert and cautions people to be wary of suspicious-looking individuals and abandoned packages when the risk is deemed high. After London's attacks, France raised the alert to red, the second-highest level.

But experts concede it's virtually impossible to fully protect any city — certainly not one the size of London, whose 8 million residents rely heavily on public transportation.

Paris couldn't, though it tried.

For months, police and soldiers armed with submachine guns took up positions at train and subway stations as well as at major monuments such as the Eiffel Tower (search) and the Arch of Triumph.

Green-jacketed transit agents watched subway passengers with piercing stares, searching for a telltale lump beneath an overcoat or an unusual bulge in a shopping bag. Trash piled up after police sealed garbage cans; today, in place of cans, there are mounted metal hoops that hold clear plastic bags.

Immigration authorities conducted several million identity checks, often singling out North Africans in a campaign that drew complaints from human rights groups.

None of that stopped the bombings, which included a December 1995 blast at a subway station in the heart of Paris that killed two people and seriously wounded 35.

"We can't prevent all bombings," said an exasperated Jean-Louis Debre, France's interior minister at the time. "Because life goes on. Because there are millions and millions of people who each day take subways, trains, buses and cars."

London has plenty of painful experience of its own.

During the worst of the bombings by the Irish Republican Army (search) in the 1970s and 1980s, bus passengers were forbidden to leave bags in luggage racks while they sat elsewhere on the bus. Trash cans were sealed, and there are few if any at most subway stations.

As British authorities vowed to redouble their vigilance, terrorism experts tempered expectations that people will soon feel safe again.

"Any society, no matter how well prepared, is vulnerable to a sophisticated terrorist with good intelligence," conceded Charles Blackmore, who heads Vance International Ltd., a British security and intelligence company.