Families of the three bombing victims gathered for the wreath-laying ceremony Tuesday morning. Boston Mayor Martin Walsh and Governor Deval Patrick were among those in attendance for the day dedicated to the victims of the twin bombings as well as the first responders, doctors and nurses who aided them.

A moment of silence was observed around the city at 2:49 p.m. local time (1849 UTC), the moment when the first bomb went off.

President Barack Obama marked the event with a private moment of silence at the White House.

"Today, we recognize the incredible courage and leadership of so many Bostonians in the wake of unspeakable tragedy," Obama said in a statement. "And we offer our deepest gratitude to the courageous firefighters, police officers, medical professionals, runners and spectators who, in an instant, displayed the spirit Boston was built on - perseverance, freedom and love."

Vice President Joe Biden led a tribute at the Hynes Convention Center, just a short walk from where two bombs made of explosives-packed pressure cookers detonated near the finish line of the Boston Marathon on April 15, 2013.

"You have become the face of America's resolve ... for the whole world to see," Biden said, adding that the "twisted, cowardly terrorists" who carried out the bombings hoped to scare Americans into abandoning "what we value."

"It infuriates them that we ... refuse to yield to fear," Biden said. "America will never stand down, we own the finish line."

Twin bombings

The attacks were allegedly carried out by two brothers of Chechen descent who had lived in the US for years. The pressure cooker bombs sent metal fragments into the crowd, killing Martin Richard, Krystle Campbell and Lu Lingzi, while many of the wounded lost limbs. Massachusetts Institute of Technology policeman Sean Collier was killed in the aftermath of the blasts, as suspects Dzhokhar Tsarnaev, now 20, and his 26-year-old brother Tamerlan, led police on a four-day manhunt.

Tamerlan died after exchanging fire with police, while Dzhokhar was wounded and captured. He will go on trial in November and could face the death penalty if convicted. Dzhokhar, a naturalized US citizen, has pleaded not guilty to 30 federal charges related to the bombings, including using a weapon of mass destruction resulting in death.

"One year later, we also stand in awe of the men and women who continue to inspire us - learning to stand, walk, dance and run again. With each new step our country is moved by the resilience of a community and city," said Obama.

This year's Boston Marathon will be held April 21. Thousands more runners have been allowed to register this year, bringing the total to around 36,000 participants.

Security has been boosted for this year's race. Spectators will be required to go through metal detectors, and a "no-bag policy" is being enforced. The bombs from last year's attack were hidden in backpacks.

dr/msh (AFP, AP, Reuters, dpa)