But when Anthony Williams, Democratic mayor of the district, said last week he would support more surveillance systems in the city's public areas following the London attacks, it drew a surprisingly harsh backlash from civil libertarians and some members of city council who said the systems were both intrusive and ineffective.

“These cameras are pigs in the poke. They do not prevent crime,” said Johnny Barnes, executive director of the DC branch of the American Civil Liberties Union. “The point is, does it make sense to spend 20 per cent of your budget, as they did in London, that didn't help those poor souls, the 54 that died. When measured against this tremendous invasion of privacy, it just isn't worth it in our view.”

But the police force said adequate measures would be taken to ensure the camera systems were not abused.

“We're using them much the same way London had if there is an incident, to allow us to respond quickly, and to have evidence for further investigation," police spokesman Kevin Morison said, adding that the camera systems were used only during big events or during heightened terror alert status. He also said the protests were often overblown because in any case the public cameras were only focused on public space downtown. “When you're walking around downtown, for every one DC government camera there are probably 20 private cameras that pick you up,” he said.

Unlike many European cities, the US has resisted the extensive deployment of surveillance cameras. Washington DC was one of the first, deploying its first camera system on September 11, although it had been developing it in anticipation of anti-globalisation protests at the World Bank and International Monetary Fund meetings that month, rather than terrorist attacks. It expanded the system in 2002, becoming the first US city to have the capability to use live digital feeds of busy areas sent to a central command post.

By 2004, Chicago, Baltimore and Houston followed suit, installing surveillance cameras in “high-risk” areas. Since the London bombings, San Francisco has responded by removing all garbage cans from its rail system, while Atlanta has trained several hundred police officers, according to a report by Newsday. New York City has faced pressure to implement surveillance systems in its transit lines, but instead has responded to the London attacks by placing a uniformed officer on all subway cars.

For Washingtonians, though, their vulnerability is seen as a fact of life.

“I really wouldn't have a problem with [surveillance systems]. I don't think it really matters,” said Christa Faern, a DC commuter who travels by rail every day to work. “[The terrorists are] just going to find a way around it anyway.”

Jack Riley, a homeland security expert with the Rand Corporation, warned against implementing new sweeping policy measures in the days after an attack, but said it was vital that policymakers piece together a “national transportation security strategy” so that “we could start understanding where that marginal dollar should go”.

Cameras and greater patrol capabilities suchas those that have beenproposed in Washington have been useful law enforcement tools in the past, he said.

“Frankly it is a very open question about how much investment is needed and what the pay-off will be. The exact wrong time to start developing an investment strategy is after an incident.”