Speaking at the Olympic stadium at the start of a week of test events that coincide with a major Olympic security exercise in the city , Coe assured reporters that the right balance would be found.

"We will have contingencies in place for all possible eventualities," he said.

"That's exactly the testing process that is taking place now...but let me make it really clear - the challenge of delivering the right level of security at an Olympic Games is not just simply about how can you lock down everything.

"We are inviting people to come to London to celebrate the greatest sporting event that this country has ever seen. We are not inviting people to Siege City."

Some residents of an apartment block overlooking the Olympic Park complex in east London have been alarmed in recent days to discover that their building is a proposed site for a rooftop missile battery.

Others have questioned the deployment of missiles in city centre locations, arguing that they pose an unnecessary risk to civilians, give terrorists more respect than they deserve and dent the Olympic spirit.

Military chiefs say this week's manoeuvres involving members of the air force, soldiers and sailors are necessary contingency measures for what will be Britain's biggest peacetime security operation.

Coe said organisers would be guided by the experts on security and were satisfied that "the right people are making the right decisions".

"There is no appetite for risk," he added. "We want to ensure these Games are safe and secure but at the same time we are not presenting a vision of London that is a city under siege, because that's not what we are."

Access to the Park on Wednesday was through airport-style security with liquids, aerosols and gels prohibited in quantities of more than 100ml, while bags were put through scanners.

Police with sniffer dogs could be seen carrying out checks around the stadium area as spectators flowed through the gates for a range of test events.

LOCOG's head of venues and infrastructure James Bulley told reporters that drinking water would be freely available at Games time and spectators would be encouraged to fill their empty bottles once inside the Park.

On a lighter note, Coe had to fend off another question about water when asked about heavy rains in south-east England and whether he regretted not providing more cover for spectators at some temporary venues.

He said he was confident organisers had got the balance right in aiming for a sustainable and environmentally friendly Olympics and the weeks of the Games - July 27 to August 12 - were statistically the lowest for rainfall.

"These venues have coped extremely well," added Coe. "We've had a couple of very minor issues but on a scale of one to 10 they are hardly on the screen."

(Editing by Clare Fallon)

• A Permanent Economic Emergency