Providing security for the 2012 London Olympic Games will be no easy task.

It is expected that 120 heads of state will visit Britain during the games. About 450,000 people are likely to be accredited to access the 46 Olympic and Paralympic sports venues, all of whom will need to be vetted for security. An estimated 10 million people—equivalent to the population of Michigan—will attend Olympic events throughout Britain, and approximately 10,000 athletes will participate in the games.

As shown by events this week in the United Kingdom, homegrown terrorism from Islamic extremists is probably the biggest threat to the games.

A routine traffic stop last Saturday uncovered weapons that led to the arrests of seven more individuals today on the suspicion of terrorist offenses. This story is still developing, and it is not clear if the seven arrested had the Olympics in their sights, but in any case, this is part of a worrying trend of homegrown terrorism in the U.K.

Yesterday was also a busy day for the U.K.’s security services and law enforcement agencies.

Smoke emanating from a bag on a bus carrying 50 passengers sparked a massive counterterrorism response and closed one of the U.K.’s major motorways for nearly four hours. According to press reports, the response involved 16 fire engines, 15 police vans, 12 police cars, 10 undercover police cars, an ambulance, and incident control vans. In one picture alone, The Sun reported that it counted 24 armed cops, six soldiers, seven firefighters, eight uniformed police, one attack dog team, and three bomb-sniffing dog teams.

What caused this commotion? An electronic cigarette costing only £6 ($10). Thankfully, a false alarm.

There was a more worrying incident yesterday. After months of surveillance and a recent increase in “chatter,” U.K. authorities arrested six individuals allegedly planning terrorist attacks in the U.K.

Among those arrested was a British convert to Islam named Richard Dart, who used to work as a security officer for the BBC. Dart was seen in a video last year going by the name Salahuddin Al Britani and threatening the royal family.

Another suspect worked for the Metropolitan Police for two years. All six were arrested in London, and two were actually arrested in their home less than a half-mile from the Olympic Park.

British intelligence sources believe that there are at least 200 potential terrorists actively planning suicide attacks in the U.K. Rather than targeting Olympic venues, where the security presence will be extremely high, it is thought that potential attackers will focus on less secure areas with large crowds, such as train stations and open-air television screenings.

British intelligence services are some of the best in the world, and the U.S. should be supporting U.K. authorities on all levels with full access to intelligence sharing and counterterrorism cooperation. As recent events have shown, Olympic security will be no easy task.