LONDON - British police have begun a yearlong program of Olympic security tests, conducting sweeping searches of London 2012 venues and practicing defensive counterterrorism tactics and lockdown techniques.

The exercises described Tuesday are part of a broad range of tests to check how police, government departments and emergency services would respond to any problems with the games. They are a part of a multifaceted security operation aimed at creating confidence in the safety of London's Olympic venues.

"This is the biggest peacetime policing operation ever put together," said Assistant Commissioner Chris Allison, who is leading the massive operation that includes 11 police departments and up to 12,000 police officers on duty a day.

Security has always been a critical concern for the Olympics since the slaying of 11 Israeli athletes and coaches at the 1972 Munich Games. The London Games are seen as a high security threat in particular, as the Olympics offer a ready platform for any terror group intent on wreaking havoc at live events broadcast worldwide.

London has already suffered the consequences of terrorism. Homegrown suicide bombers attacked London's transit network in 2005, killing 52 commuters the day after London was awarded the Olympic games.

The British government has repeatedly said the national terror threat will remain at the "severe" level during the Olympics, meaning an attack is "highly likely."

"We're planning against a severe threat level for 2012," Allison said. "We're doing lots to minimize the chance for that happening."

The Olympic security program builds on testing that has already taken place, and doesn't include a separate program by government agencies and emergency services to test incident responses.

The Olympic program includes command-post exercises in September and December and a live counterterrorism exercise in spring 2012.

British authorities have refused to comment on whether any specific threat has been made to the London Games. Home Secretary Theresa May refused to even entertain a question Tuesday on whether British officials had received any intelligence suggesting that slain al-Qaida chief Osama bin Laden had planned to target the games.

Underscoring the security preparations, British officers dangled on ropes from the rooftop of the Olympic Stadium in a drill two weeks ago, searching for suspicious objects placed in spotlights. Detector dogs sniffed out mock explosives and officers carried out systematic hand searches.

The searches are also intended to work on timing - or how long it takes to search a venue.

"These procedures are helping build confidence they will not have to face the unknown in 2012 and the venues are secure before use," Allison said.

However, Allison said authorities were anxious to not make security efforts oppressive. He said visitors to the Olympic park and sports venues would encounter security somewhat similar to what they would face at an airport.

Political leaders, meanwhile, promised that money would be in place to fund the Olympic security operation.

May told reporters the 600 million-pound ($965 million) Olympics security budget is being protected from spending cuts and won't be affected by the tough measures aimed at reducing Britain's burgeoning budget deficit.

The 2012 London Summer Olympics begin July 27 and run through August 12.