A day after a security scare closed a major highway for hours but turned out to be a false alarm, British police officials reported Friday that they had seized seven men under counterterrorism laws, bringing the total number of arrests announced in the past 24 hours to 13.

The unusually high number of detentions reflected concerns that when the 2012 Olympics open in London in three weeks, they could become the highest-profile targets since the 2001 attacks in New York and Washington.

The authorities’ readiness to respond in strength to perceived threats was illustrated Thursday when police closed one of Britain’s major intercity expressways in a seven-hour alarm that drew in an armed counterterrorism unit, police helicopters, fire engines, ambulances and scores of emergency workers.

In the end, the cause of the scare seemed prosaic: A man traveling on a bus to London tried to use an electronic cigarette as an aid in quitting, causing another passenger who saw vapor to alert police by cellphone.

Earlier the same day, though, police in London announced the arrest of six people on suspicion of committing, preparing or instigating acts of terrorism and three of them were seized in east London close to the location of the Olympic stadium and village.

On Friday, police in Britain’s Midlands region around Birmingham announced the arrests of seven people who had been tracked down after a routine police inspection of an impounded vehicle uncovered “firearms, offensive weapons and other material” that prompted police “to take action to trace and arrest the driver, passenger and others suspected of being involved,” according to a police statement.

Three men between 23 and 27 years old were arrested in the Sparkhill neighborhood in Birmingham on Tuesday, three more suspects between 22 and 24 years old were arrested in other areas Wednesday evening, and a 43-year-old man was seized in West Yorkshire late Thursday, police said.

While police officials have not drawn any formal links between the latest arrests and the Games, the risk of a terrorist attack in the coming weeks has been a major concern for British officials – and for the nearly 200 nations and territories that will be competing – since the Olympics were awarded to London in 2005.

The concern was amplified when Islamic militants bombed London’s transit system the day after the Games were awarded. The bombing killed 56 people, including the four bombers, in what was Britain’s worst terrorism attack.

Ten days ago, Jonathan Evans – the director general of MI5, Britain’s domestic security service – offered a heavily qualified statement of reassurance. He said officials with Britain’s terrorist-alert system had estimated the threat of a terrorist attack as “substantial,” the third-highest rating on a scale of five, meaning that a strike was “a strong possibility.”

But in a rare public address, Evans said, in effect, that years of experience in monitoring terrorism threats and rounding up potential attackers since 7/7 – as the attacks of July 7, 2005, are known – and a much-improved system of international cooperation had vastly improved the chances of heading off a terrorist attack on the Olympics.

“There is no such thing as guaranteed security,” he said. “But I think we shall see a successful and memorable Games.”

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.