Two days before the 2004 Olympics begin, about 70,000 Greek security forces are patrolling Athens and competition venues scattered around the country, while 1,000 security cameras and a couple of blimps are keeping an electronic eye on the proceedings. Greek fighter planes and NATO surveillance aircraft are guarding the skies. Patriot missile batteries have been installed to confront potential terrorist threats from above.

In the midst of international terror concerns, Greek government officials say they have gone to extraordinary lengths to ensure the safety of the Games. A seven-nation advisory group and NATO have lent their expertise and resources in an unprecedented cooperative effort of a scale unimagined when Athens won the Games in 1997.

But Greece alone has been left to pay the bill -- $1.5 billion, nearly triple the original projections. Greek Prime Minister Costas Karamanlis said Greece "asked and happily received the assistance, good advice and contributions of all our allies," but no nation or group offered to defray the costs, which traditionally have been borne by the host nation.

"No country in the world may face the security challenges of our era by itself," Karamanlis said during an interview at his residence. "As long as we organize collective, big, international events and as long as security is a common thread for all of us, we've got to find a balanced approach to share, or at least be part of, the bill."

The price tag is more than four times the security costs for the 2002 Winter Games in Salt Lake City and six times that spent for the 2000 Summer Games in Sydney. That, coupled with construction delays and other unexpected expenditures, has led to a cost overrun of some $1.7 billion, pushing Olympic-related expenses to at least $7.2 billion, according to recent estimates from government officials. The costs have contributed to a national budget deficit that has risen above the maximum permitted by the European Union.

"Greece has to make an effort to become an economically competitive country," Karamanlis said. "This, to an extent, has been aggravated, been burdened, by the cost of the Olympic Games."

The International Olympic Committee provided $905 million to the Athens Organizing Committee through its standard host city contract, though no moneys were earmarked specifically for security. The bottom line for these Games is difficult to predict given the cost overruns and sluggish ticket sales; as of Wednesday, more than 2 million tickets remained.

The centerpiece of the security apparatus is a $312 million command-and-control center built by a U.S. consortium that collects a constant stream of video, audio and other data beamed from around Greece, including eavesdropped telephone calls and conversations on street corners. Construction of the system was plagued by delays, however, and became functional only weeks ago.

Last week, Greek Public Order Minister George Voulgarakis said that the security measures were fully operational and working well. But others involved in the project said many kinks still had not been worked out and that the communications network was not as comprehensive as originally planned.

Although the security system is described as highly advanced, Greek officials have had little time to run tests on the network or to become familiar with its capabilities.

"The technology side is going to be very hard to manage, even if it is working perfectly," said Philip Giraldi, a former CIA official who oversaw security preparations for the 1992 Summer Olympics in Barcelona. "How do you sort out 120 different video images that you're watching at once? I think the technology has its limitations and that your security is going to very much depend on the security that you have on the ground."

The Olympic security operations are geared toward preventing a large-scale terrorist attack intended to produce mass casualties, such as threats from a hijacked airliner, chemical weapons or hostage-seeking gunmen, according to Greek and Western officials.

Counter-terrorism officials from Greece and elsewhere in Europe said there have been no signs or specific intelligence indicating that such attacks are being planned. More likely, they said, was the chance that the Olympics would be targeted by one of Greece's many small anarchist or radical political organizations that regularly ignite pipe bombs and other small explosives in Athens.

On May 5, precisely 100 days before the Opening Ceremonies here, a police precinct in Athens was dynamited by a group calling itself Revolutionary Struggle. No one was injured in the blast, but in a statement delivered to a newspaper a few days later, the group said it carried out the bombing to show the "vulnerability" of Greece's security measures and to protest the coming influx of "wealthy" Western visitors and business leaders for the Olympics.

Mary Bossis, a Greek terrorism expert and an adviser to the government, said it was an embarrassment that Greek officials had been unable to make any arrests in that case despite spending so much on security measures and intelligence gathering for the Olympics.

"We have all these very expensive toys floating all over the place, but they have not proven their worth at all in terms of finding domestic terrorists," Bossis said. "There are a number of these groups and they have voiced their disagreements with the Olympics very loudly, but so far I haven't seen any ability by the authorities to arrest or identify any of them."

Olympics officials said Wednesday that they have been conducting emergency drills -- including practice evacuations of the newly built Olympic Stadium -- and remain confident that they have done everything possible to ensure the safety of the Games.

"Athens is ready," Gianna Angelopoulos-Daskalaki, chief of the Athens Organizing Committee, said Wednesday. "What we promised, we delivered."

Even so, some countries are taking security matters into their own hands. The United States, Australia, Israel and Britain are quietly planning to bring armed security guards to provide individual protection for athletes and dignitaries, in spite of a Greek ban on foreigners carrying weapons, according to officials and media reports from those countries.

The Times of London reported on Tuesday that British officials were transporting handguns to Greece in diplomatic pouches for its 130-member security contingent from Scotland Yard.

A U.S. embassy official declined to answer questions about whether U.S. security agents would be armed. Greek government leaders have denied the existence of any agreements to allow foreigners to wield guns.

For months, U.S. officials have pressed the Greek government to allow U.S. security personnel to assist in safeguarding the U.S. team, made up of more than 500 athletes. Greek and U.S. forces will accompany U.S. athletes and those from other nations on the buses that shuttle them to events. Though the vast majority of U.S. athletes are expected to stay inside the heavily guarded Olympic Village, those that don't will be protected by security details.

At least 100 FBI and State Department security personnel will be in Greece during the Games. The U.S. Olympic Committee has provided emergency masks for the U.S. Olympians, urged them to travel in groups when they go about the city and encouraged them to show restraint when wearing red, white and blue or Team USA gear outside of the venues or Olympic Village.

"Nobody can guarantee absolute security, but everything humanly possible to be done has been done," Karamanlis said. "I'm very confident the Games will be both successful and secure."

A Greek soldier patrols in Athens. Other countries will bring armed security guards for protection.With Patriot missiles in place, the Olympic security operations are geared toward preventing large-scale attacks intended to produce mass casualties.