More than 1,000 troops who had been on standby to bolster security at the Olympic Games have been deployed.
The Cabinet Office said the 1,200 troops were to be used because ministers were clear that “we should leave nothing to chance”.
Earlier this month an extra 3,500 personnel were drafted in after G4S said it was short of security staff.
Meanwhile, spectators have attended a preview of the opening ceremony of the Games, which will take place on Friday.
About 62,000 are set to attend the ceremony at the Olympic Stadium in Stratford, east London, which is expected to be watched by an audience of billions worldwide.
Thousands who attended the technical dress rehearsal on Monday night were urged to keep the details secret and not spoil the surprise for others by circulating images or videos.
Danny Boyle, the ceremony’s artistic director, reportedly asked the audience to “save the surprise”.
Olympic organisers Locog said the technical rehearsal for the opening ceremony “went very well”.
“The reaction from the attendees has been overwhelmingly positive on social media and crucially they are still helping us to save the surprise for the rest of the nation on Friday,” it said.
In other developments on Tuesday:
Transport for London has apologised after people leaving the rehearsal were delayed getting home because the Central Line, which serves Stratford, was partly suspended
The British Olympic Association says it has asked triple jumper Phillips Idowu to supply details about his injury ahead of the Games after the 2008 Olympics silver medallist pulled out of the Great Britain athletics team’s training camp
Heathrow is set for its busiest day for the Olympics so far as it prepares to handle about 217,000 passengers, including 1,200 athletes and coaches and more than 3,000 other Games-related arrivals
Motorists have faced long queues on routes where pre-Olympics modifications have been made to road layouts
It has emerged that Big Ben will chime 40 times on Friday as part of a mass bell-ringing event marking the start of the Games
The Olympic torch is travelling from Kingston, in south west London, to Ealing, west London, as it continues its tour of the host city’s 33 boroughs.
People who attended Monday’s rehearsal reported that the hashtag “#savethesurprise” was emblazoned on giant screens inside the Olympic Stadium.
It later trended globally on micro-blogging website Twitter.
Most people appeared to obey the appeal for secrecy, although there have been reports of some images being posted on the internet before being removed shortly after.
Many took to Twitter to show their support after the rehearsal.
“I am now sitting on my seat in the Olympic Stadium. But I shall #savethesurprise and not tweet details of the ceremony. Sorry,” tweeted Dan McNeil.
And many gave positive reviews of the ceremony on the website.
One person who was in the audience, Pete Hendrick, tweeted: “If you’ve got plans Friday night, cancel them. Opening ceremony is out of this world. Danny Boyle, I salute you.”
Chris Golds, another audience member, said the event was “breathtaking” and “awe inspiring”.