In briefings during the prelude to the Games, senior MI5 officials have elaborated on that theme. Their message has been that an international counterterrorist team involving agents from MI5 and its sister agency MI6, Britain’s secret intelligence service – and from the FBI, the CIA and other Western intelligence services – has built as effective a shield against an attack as is practically possible.

Still, events Thursday and Friday appeared to signal the fine balance of the preparations, suggesting that Britain, for all its efforts, remains deeply apprehensive about the terrorist threat.

After the arrests Thursday police said no attack was imminent, which appeared to align with what MI5 officials have said about a succession of terrorism-related arrests in Britain this year involving at least 23 suspects, most of whom, like those arrested Thursday, have been British citizens.

Officials say the imminence of the Games has caused security agencies to move earlier to arrest suspects than they would have otherwise. This has meant, they said, that many suspected plots have been interrupted at an early stage, before any bombs have been built, weapons acquired or targets selected.

In recent years, British officials have warned that a wide network of Islamic militants’ cells have been established across the country. Some of them, the officials say, are actively plotting terrorist attacks, often benefiting from the cultural, political and religious alienation that has been common among Britain’s 1.5 million Muslims.

Recent police raids have included the arrest of two men last week on the suspicion that they were plotting an attack on the Olympics canoeing site on the Lea River, on London’s outskirts. In May, seven men were arrested on suspicion of financing terrorist plots with money earned from the smuggling of the stimulant khat, which is widely used in Somalia and countries on the Arabian Peninsula, to users in Canada and the United States. A few weeks earlier, three men from Birmingham were arrested at Heathrow Airport in London after a flight from Oman and accused of possessing “articles and documents” for use in international terrorism plots.

“In back rooms and in cars and on the streets of this country, there is no shortage of individuals talking about wanting to mount terrorist attacks,” Evans said in his speech. “The threat is real.”

Evans’ aides say the security force of 27,500 people that will be deployed to protect the Games and a security budget of nearly $1 billion should make it difficult for attackers to reach the athletes, spectators and Olympics sites. Any would-be terrorists, the aides said, may calculate that they have a better chance of success by attacking elsewhere in Britain.