LONDON — To understand the gargantuan security operation that authorities have prepared for the Olympic Games that open Friday — nearly twice as many military troops are on the streets of London as Britain has deployed in Afghanistan, for starters — it helps to remember what happened the morning after the city won the right to host these games seven years ago.

On July 7, 2005, homegrown militants detonated bombs in the Underground subway and on a passenger bus, killing dozens in one of the worst terrorist attacks in the country's history.

While there's no evidence that attack was linked to the Olympic bid, it's had an unmistakable impact on planning for these Games. The 10,000 athletes competing for medals will be dwarfed by a contingent of more than 36,000 soldiers, police officers and private security staff, backed by U.S. law-enforcement agents, thousands of closed-circuit cameras, unmanned drones, at least six missile batteries positioned on rooftops in East London and the Royal Navy's largest warship, the HMS Ocean, floating in the Thames.

"Lockdown London," read one headline in the Guardian newspaper.

Authorities stress there's no specific threat to the Games, but any attempt to deflect attention from the military-level planning evaporated last week, when the private security firm G4S shamefacedly admitted it would fail to deliver all of the 10,000 personnel it had promised to guard Olympic venues.

On Tuesday, British officials said they would activate an additional 1,200 military personnel to fill the shortfall, bringing the total of British troops who will work the Olympics to 18,200. Britain deploys about 9,500 in Afghanistan.

"On the eve of the largest peacetime event ever staged in this country, ministers are clear that we should leave nothing to chance," Jeremy Hunt, the culture secretary, said after a Cabinet committee meeting. "The government continues to have every confidence that we will deliver a safe and secure Games."

The measures have prompted worries from civil-liberties advocates about heavy-handed policing, excessive surveillance and whether British officials have taken precautions too far. In one case Friday, 25 police officers responded to a protest in Trafalgar Square involving three people who were enacting a play critiquing the environmental record of chemical companies that are Olympic sponsors. Six people were arrested for "criminal damage" after some green custard spilled, according to news reports.

"I don't think anybody is under the illusion that London isn't a potential target, but as people start to see thousands of troops, military installations, a huge increase of civilian security, they will start to wonder if the authorities have lost sight of the proportionality in their response," said Nick Pickles, director of Big Brother Watch, a civil-liberties group.

Already, by the group's reckoning, London is one of the most heavily surveilled cities in any democracy, with an estimated 80,000 closed-circuit cameras watching over the public transport system and countless more installed on roadways and in private businesses. An additional 1,800 cameras have been planned in the Olympic Park and athletes' village, Pickles said, adding that authorities haven't said whether the cameras will be taken down after the Games.

"Once all the infrastructure is put in place," Pickles said, "there's a risk that that's the legacy of the Olympics: that everyday life in London becomes far more restricted and inhibited by a security apparatus which people have come to see during the Games, that might not necessarily be dismantled."

At a time of sharp cuts in social services, the budget for Olympics security has doubled to more than $850 million. Security experts defend the measures, given the size of the target and the fact that this is the 40th anniversary of the terrorist attack on the Munich Olympics.

"The vast majority of the British public would rather be safe than sorry," said Margaret Gilmore, a homeland-security analyst at the Royal United Services Institute, a defense and security research center. "They do care about this whole fiasco with G4S, but while some might worry about the cost, the vast majority really just want to be safe."

A judge this month rebuffed residents of one East London apartment block who sued the government after they were told that surface-to-air missiles would be positioned on the roof of their building, about two miles from the Olympic Stadium, to enforce a no-fly zone over the area. The judge ruled the British Defense Ministry didn't need to ask the residents' permission.

Five other buildings also have been named as sites for rooftop missile batteries, including the Lexington building on Fairfield Road in the Bow section of East London, less than one-half mile from Olympic Park. The missiles aren't visible from the street, however, and residents who were interviewed Tuesday about their apartment's newfound counterterrorism role reacted with the studied nonchalance that's practically a British pastime.

"I'm indifferent," said Peter Fawcett, 29. "Some people were annoyed, and they organized meetings to protest it. But we're so close that we were going to be affected anyway. I guess I'd sooner have it there and not need it than need it and not have it."

As for the security onslaught, Fawcett's companion, Clare McAnaney, 23, offered that there was a positive side to seeing thousands of large, uniformed men on the streets of central London.

"The women love it," she said.