The London 2012 chairman, Lord Coe, has promised the capital will not become a "siege city" during the Games, insisting that recent announcements over the deployment of troops and military hardware will be balanced by a sense of "proportionality".

Organisers have long acknowledged that while they will not be able to compete with the scale and budget of the 2008 Beijing Games, they hope London will be remembered for an ebullient welcome and electrifying atmosphere.

That will have to be balanced against a ballooning security operation both inside and outside the venues, which this month trumpeted the deployment of a warship on the Thames and surface-to-air missiles on standby.

Initial estimates of 10,000 security guards within venues have swelled to 23,700, including 7,500 military personnel. Meanwhile, 12,000 more police and 6,000 military personnel will be on duty in the streets outside the venues.

But Coe – the head of the London Organising Committee of the Olympic and Paralympic Games – promised security would not be overbearing and would be mindful of the need for welcoming atmosphere for ticketholders from the UK and abroad.

"There has to be proportionality here. You don't want people coming to London thinking they've walked into siege city, being filmed every 20 paces they take and being bundled off pavements," he said. "It's certainly not what you're going to get legacy tourism from.

"There is no appetite for risk. Everybody knows this is a complex, complicated world and this is a big global city. But we also want people to come here and leave feeling they've had an extraordinary time and want to come back."

Coe said he did not want people "kicking their shoes off on the plane and thinking: 'My God, we've just got through it and we're now going home, thank goodness'". Instead, he said: "I want people to leave in the way I left Sydney [in 2000], thinking: 'My God, can there really be life now after that?'"

In an interview with the Guardian to mark the dawn of Olympic year, Coe said he was confident public enthusiasm would build towards a "Halley's comet" moment.

"I think people will realise they're in a very special year. It's a bit like Halley's comet, it doesn't come around that often and everything is in alignment," said Coe, who also rejected recent criticism from the National Audit Office over budgetary issues.

"There is nobody alive who is probably ever going to see the Games come back to this country. That is just the nature of the way that the world is becoming a bigger place and more countries are able to deliver. This is the moment of truth and that's why all our teams have been doing what they're doing for as long as they have been."

Locog has also been criticised for accepting £41m from the £9.3bn public funding package to double the budget for the opening and closing ceremonies. Coe said it was the right decision.

"I don't believe we want to be sitting in a stadium with all the eyes on us and ignoring that opportunity. Over the long haul, people would look at that and say it was a massive opportunity missed. It was the right decision," he said.

"This is unique. This is in front of 4 billion people, a stadium audience of probably 100 heads of state. I can't think of a better way to energise all the things we're doing out there through any number of agencies, all the things that government and all our agencies out there selling the country at a particularly difficult time are trying to do."