By Richard Galpin
BBC Athens correspondent
Thousands of members of the security forces are being deployed in Athens and other cities on Thursday, marking the beginning of the full security operation to protect the Olympic games, which start in mid-August.
Sports venues and other critical buildings for the games will be gradually sealed off over the next few weeks and searched for explosives.
Greek forces have been training in different scenarios
After years of planning and training, the biggest security operation in the history of the Olympics, costing up to one billion euros (£600m), is now being implemented.
Almost 40,000 policemen and coastguards, plus thousands of troops, will be deployed to guard the main sports venues, the Olympic villages, Piraeus port, and any other potential terrorist targets.
Air and sea patrols are also being stepped up around the country.
But there is less certainty about when the so-called lock-down and sweeping of the sports venues will begin.
This is a critical part of the security operation, when access to the venues will be tightly controlled and they will all be thoroughly searched using sniffer dogs and hi-tech equipment to ensure no explosive devices have been planted.
Government officials say all this will be completed by the target date of mid-July.
But, for example, at the main Olympic complex, there is still a massive amount of building work going on, with huge lorries constantly moving in and out.
And there is still not a perimeter fence around the whole area. According to Olympic officials contacted by the BBC, this complex will not be sealed off and checked until just days before the opening ceremony.