LONDON — Each day, the average Londoner is filmed 300 times as he or she walks the children to school or takes the train to the office. Britain has 4.2 million closed-circuit surveillance cameras, one for every 15 people in the country, security experts say.

In a nation that, like the U.S., worries about the potential for terrorist attacks as well as regular crime, most people are hardly bothered by the lack of privacy. But a new round of government proposals — to dramatically expand surveillance and data collection and to strengthen other anti-terrorism measures — has some public officials alarmed.

They’re warning that the government must not go too far in this city where George Orwell set “1984,” the famous novel about the dangers of an all-seeing “Big Brother” government.

“We need to take very great care not to fall into a way of life in which freedom’s back is broken by the relentless pressure of a security state,” warned Sir Ken Macdonald, Britain’s director of public prosecutions, in a speech last week.

Surveillance data have helped his office successfully prosecute 90 percent of terrorism cases, he said. But technological advances are giving the government the ability to track people “every second of every day, in everything we do,” he warned. “We should take very great care to imagine the world we are creating before we build it. We might end up living with something we can’t bear.”

London has an ever-growing number of discreet security cameras keeping an eye on schools, trains and city streets, particularly in high-crime areas. And British spy agencies, like those in the U.S., also have access to telephone records.

But the government wants to begin keeping databases of e-mails sent, calls made on Skype, exchanges on social networking sites such as Facebook, chats on gaming sites, communications made through eBay and a variety of other Internet interactions.

In addition, the government proposes requiring registration of all cell phones in the country — today more than half are unregistered prepaids — and hopes to issue a national identity card for everyone living in Britain, with details stored on a central database. It also proposes giving each child an identity number that will follow them through life.

Jacqui Smith, Britain’s home secretary, calls such changes “vital” to anti-terrorism efforts. She emphasizes that the content of e-mails and phone calls would not be recorded. But with more and more people — including terrorists — exchanging information through a multitude of “chatting” options, tracking the flow of communication on the Web is crucial, she said.

Privacy protections and civil liberties have eroded around the world in recent years as nations struggle to balance cherished freedoms with efforts to combat terrorism. For example, Britain’s government, faced with a revolt by legislators, recently dropped plans to extend detention of terrorist suspects without charge from 28 to 42 days.

But, in a separate initiative, it appears to be pressing ahead with plans to issue national identity cards that would include chips holding “biometric” data like fingerprints and photos. A former head of Britain’s MI5 domestic spy agency last week called such an effort an overreaction to terrorist threats.

“The British, like the Americans, know there are terrorist cells out there that want to cause mayhem. But they don’t yet know how to strike a balance between doing what is absolutely necessary to stop those attacks and preserving the civil liberties that are the essence of Western civilization,” said Robin Shepherd, a foreign policy expert at Chatham House, a leading London think tank.