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

On July 7, 2005, homegrown militants detonated bombs in the city’s 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 American 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 that there’s no specific threat to the games, but any attempt to deflect attention from the combat-grade planning evaporated last week, when the private security firm G4S shamefacedly admitted that it would fail to deliver all of the 10,000 personnel it had promised to guard Olympic venues.

On Tuesday, British officials said they’d activate another 1,200 military personnel to fill the shortfall, bringing the total of British troops who’ll work the Olympics to 18,200. For comparison, 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 last 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. The police arrested six people for “criminal damage” after some green custard spilled to the ground, 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, the 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.