Hundreds of thousands of runners and spectators converged Sunday on the first major international marathon since the Boston bombings, turning the streets of this sprawling capital into a living tribute to the victims of last week’s attack on the other side of the Atlantic.

Under the watchful eye of boosted security forces, large crowds turned out for the London Marathon, with many calling their attendance a symbol of the determination of Londoners to remain unbowed by the tragedy in Boston. Lanky runners — including elite athletes who run the global circuit as well as a host of local amateurs — wore black ribbons pinned to their T-shirts in remembrance of the Boston victims. Some carried banners simply declaring, “For Boston.”

Before the start of the race at 10 a.m. local time, a 30-second period of silence was observed. Many runners vowed to hold their hands over their hearts in a gesture of support for Boston as they crossed the finish line.

“We are here today to show our solidarity for the people of Boston, who suffered those horrible attacks,” said Martin Ilott, 48, a British veterinarian who ran the Boston Marathon last week and was being treated for dehydration when the bombs went off at the finish line. On Sunday, he clutched an American flag as he prepared to watch his 19-year-old son run his first marathon. “You have to carry on.”

In the wake of the Boston attacks, Scotland Yard deployed several hundred more officers to ramp up race-day security and reassure the public. More than 650,000 people were expected to watch 36,000 runners traverse the iconic course that snakes its way from leafy Greenwich Park to the finish line on the Mall, a wide boulevard leading to Buckingham Palace.

A few here expressed lingering safety concerns. But on a sunny spring morning very similar to last week’s fateful day in Boston, London appeared largely able to push aside any sense of fear, with many in the crowd embracing a light-hearted, even festive mood. Men running for charities wore dresses and pink wigs. Women were decked out in blue tutus, and the Beach Boys’ “Good Vibrations” blared over the sound system as runners stretched and warmed up.

Terrorist threats have long been a reality for Londoners, many of whom still vividly recall July 7, 2005, when four bombs detonated on subway trains and a double-decker bus killed 52 people and injured more than 700.

“That fear is always there for any big public event. It’s just the age that we live in,” said Billy Bambrough, 25, a reporter from Kent, who was running the London Marathon for the first time. “It’s important, when these horrible things happen, to not change our behavior and things we do in light of it.”

Many here like Tricia Bunn, 38, an elementary school principal from Tamworth, near Birmingham, had run the Boston Marathon only six days earlier. Rather than deter runners from turning out, the events in Boston instead seemed to inspire them to run Sunday.

Bunn, who finished the Boston Marathon 30 minutes before the bombs went off, said the impact of the attack on the running world would bring an incredibly strong community even closer together.

“Everyone on that starting line will be doing it for Boston,” Bunn said, her voice shaking as she held back tears. “I think it will be the greatest message that there could be to the bombers, and to the people of Boston.”

Some here, however, felt a sense of unease ahead of the race.

“There had been this great excitement and buildup, but with what happened in Boston, it makes you just think what could happen here,’’ Debbie Georgiou, 36, said. “It’s unbelievably tragic.”

When the Georgiou family first saw the news of the Boston attack on television last Monday, Georgiou’s 11-year-old daughter burst into tears, begging her not to run. The family finally agreed that their two daughters would not attend the race, but would meet their mother after she finished the race.

“That gives me a little bit more peace of mind,” Georgiou said. “There is that fear — you can’t help it. I didn’t want anyone near the finish line. That’s a natural mother instinct.”