As tens of thousands of athletes, team officials and visitors gather in London ahead of the Olympic opening ceremony on Friday, security is paramount for the Games organizers and British authorities.

And perhaps the biggest headache of all is the challenge of safeguarding the more than 100 heads of state and foreign dignitaries who will attend the opening ceremony at the Olympic Park -- Queen Elizabeth II, first lady Michelle Obama and U.S. presidential hopeful Mitt Romney among them.

But should visitors and competitors be worried?

Security concerns hit the headlines this month when it emerged that private security contractor G4S, which was supposed to have provided 10,400 guards for the Olympics and Paralympics, would not be able to deliver.

As a result, the government is deploying 18,200 troops -- many more than planned and almost twice as many as are in Afghanistan -- in order to remedy the shortfall. About 1,200 of those were called up just this week after being placed on standby.

Nonetheless, Culture Secretary Jeremy Hunt, the minister responsible for the Games, said Tuesday that the government "continues to have every confidence that we will deliver a safe and secure Games."

The decision to call up the extra troops was down to ministers' determination to "leave nothing to chance," as they prepare for the largest peacetime event ever staged in Britain, he said.

"G4S numbers continue to rise significantly and we have every expectation that will continue to be the case," Hunt added.

With three days to go before the opening ceremony, G4S said it had around 5,800 security personnel deployed at Olympic venues, with more being trained and accredited "each day." Last week, it said it hoped to have 7,000 fully accredited by the start of the Games, still thousands fewer than it was contracted to provide.

While the recruitment failure by G4S is highly embarrassing for the company, the Games' organizers insist that their contingency planning will keep everyone safe.

Visitors can certainly expect to see far more visible security measures in place than usual at Games venues and transport hubs.

The Ministry of Defence is also guarding the games with two warships, Typhoon jet fighters, Puma helicopters and, perhaps most controversially, surface-to-air missiles on apartment buildings near the stadium, despite objections from residents.

Sebastian Coe, the chairman of the London organizing group, LOCOG, has said the only real difference resulting from the G4S debacle will be in the "mix of security" checking on those at Olympic venues.

Home Secretary Theresa May gave a similar message when she was grilled by lawmakers on the matter, saying, "There is no question of Olympic security being compromised."

Critics have been less positive, with some Labour Party lawmakers concerned that security will be inadequate or that the prominent military presence will make visitors uncomfortable.

Security officials in the United States say they are supporting the host nation's efforts during the Games.

The U.S. intelligence community has set up a 24/7 center to analyze all threat information coming in during the Olympics in London, a senior American counterterrorism official said Wednesday.

Matthew Olsen, director of the National Counterterrorism Center, told the House Homeland Security Committee that the upcoming Games "present a potential target for terrorists and other disruptive groups."

He said the United States is working closely with its British counterparts to collect, analyze and share potential threat information with the goal of making sure they can "respond quickly to prevent any possible plotting tied to the Games."

Defense Secretary Leon Panetta said last week that Washington has offered "whatever assistance" the UK needs "in order to ensure that proper security is provided."

The chief executive of G4S, Nick Buckles, was forced to agree under questioning from lawmakers that the security staffing fiasco is "a humiliating shambles for the country." The firm has also agreed to meet the cost of the extra troop deployment, which could run to $77 million.

The Home Office said that G4S was suffering from a software problem, which means the contractor cannot guarantee who will turn up where and whether guards have the right training.

The guards employed by G4S will be responsible chiefly for such tasks as providing venue perimeter security, a spokesman for the contractor said. This includes manning X-ray machines, searching people, searching vehicles and operating closed-circuit television systems, he said.

People with tickets for Olympic events have been sent e-mails giving details of the security requirements for each venue. These include airport-style limits on the size of bags that can be carried and the quantity of liquids that can be taken in.

One such e-mail reads, "Be prepared for security checks when you arrive: this will be like taking an international flight at an airport. It will be busy and you will have to queue, so get there early."