Tens of thousands of London 2012 volunteers will descend on Wembley Arena on Saturday for the first stage of training for a role seen as vital for a successful Olympics.

Some will end up holding Usain Bolt's kit as he settles down in the blocks for the 100m final, but thousands more will end up giving directions or manning car parks.

In Sydney in 2000, volunteers were hailed as one of the ingredients that made the event a success but at other Games bored unpaid staff have quit and ticket-holders have complained about volunteers' lack of local knowledge.

Paul Deighton, the chief executive of Locog, the 2012 organising committee, said this weekend's orientation event for the so-called Games Makers was a crucial part of the process. Organisers will seek to enthuse and inform 40,000 volunteers in four sessions. In all, Locog will employ 70,000 Games Makers alongside 130,000 other full-time and contract staff.

"This weekend is a really big weekend for us," said Deighton. "Making sure we have got our people and that they're all pointing in the right direction, motivated and fully trained is as big a challenge as we have, given its scale. Getting this right is really, really vital for us."

Organisers received nearly 250,000 applications and are still in the process of filling the 70,000 places available in the Olympics and Paralympics after holding selection events at eight regional centres.

Jean Tomlin, Locog's HR director, said 80,000 applicants had been interviewed and 55,000 places offered. The process will continue until all 70,000 places are filled, with a further 15,000 to 20,000 people placed on a reserve list.

"We've had a phenomenal response in terms of energy, focus and excitement. We've had 95% acceptance rates, which is unprecedented," she said.

The mass "orientation" events, which will be repeated at 32 similar exercises in cinemas up and down the country, are designed to provide an introduction to the history of the Olympics and initial training.

Deighton will be joined by the Locog chairman, Lord Coe, and Olympic rowing medallist Sarah Winckless at the London event.

The three-hour sessions would leave volunteers "feeling energised and knowing what's expected of them", said Tomlin.

More specific sessions will then be arranged depending on the role of each volunteer and the venues in which they will be working.

The volunteers, who will wear distinctive purple and red Adidas uniforms inspired by the Grenadier Guards and the Beatles, will be assigned early to specific teams and venues so they can be well-informed, said Deighton.

"If you look at past Games, people are generally very good on the smiling and welcoming and the challenge is if they can really help if you have a question that goes one level below the surface," he said.

"We want to make sure we not only have really welcoming people but that they really know what they're doing."

Tomlin said that ensuring volunteers were fully occupied and excited was important, to ensure they didn't become bored and quit. They will be offered prizes, including tickets to the opening ceremony dress rehearsal, as incentives.

Volunteers will be fed and watered and receive a free uniform and travelcard, but none of their other travel or accommodation expenses will be met.

"We've learned from previous Games that if you don't focus on that you will get attrition. We will provide rewards and recognition throughout their journey and throughout their shifts," Tomlin said.

She said the average age of volunteers was around 35, and ages ranged from 15 to 80. "Some Games you've had a very young crowd, very student-orientated. Others have had a more mature crowd. We've worked hard at having a real spread," said Deighton.

Up to 3,000 volunteers have been drafted in to assist with security inside the venues after estimates of the number of staff needed soared from 10,000 to 23,700. Tomlin said they would help ticket-holders prepare their bags and belongings for the security screening process and manage queues, but would not be involved in the screening itself.