BOSTON (Reuters) - U.S. Vice President Joe Biden, local leaders and officials and survivors of the Boston Marathon bombing shared messages of thanks and defiance on Tuesday at a tribute to the three people killed and 264 wounded in the attack exactly one year ago.

From Patrick Downes, who lost a leg when a pair of homemade bombs ripped through the crowd at the race's finish line, to Biden, speakers recalled how police officers, spectators and others on the scene reacted immediately to help the wounded amid the chaos on April 15, 2013.

Former Boston Mayor Thomas Menino, who managed the response to the attack during the final year of his two decades in office, recalled the struggles of the families of Martin Richard, 8, Krystle Campbell, 29, and Chinese national Lu Lingzi, 23, who died in the largest mass-casualty attack on U.S. soil since Sept. 11, 2001.

"You have struggled to get through the good days and the bad," said Menino, who had been hospitalized at the time of the blasts but responded to the scene against his doctor's orders.

"I know because so many of you have told me about this year of firsts. First birthday without your beloved son, first holiday without your daughter, first July 4 where the fireworks scared you."

Downes, who had been standing near the finish line with his wife when the bombs went off, causing each to lose a leg, told the crowd of about 2,500 people that he had been impressed by the city's outpouring of support for the wounded.

"We would never wish the devastation and pain we have experienced on any of you," Downes said. "However, we do wish that all of you, at some point in your lives, feel as loved as we have every day of this past year."

Following the ceremony, the crowd walked down Boylston Street, the final stretch of the marathon, in heavy rain and high winds to watch officials raise an American flag at the finish line.

At 2:49 p.m. EDT (1849 GMT), the time the first bomb went off, the city observed a moment of silence. Afterwards, churches throughout Boston tolled their bells and ships in the city's harbor sounded their horns.

'NEVER ... YIELDED TO FEAR'

Federal prosecutors contend that two ethnic Chechen brothers placed the pressure-cooker bombs in backpacks at the race's crowded finish line and three days later shot dead a police officer in an unsuccessful attempt to steal his gun.

Biden, who spoke at a memorial service for the slain officer soon after the attacks, said events like the marathon bombings or the 2001 attacks on New York's World Trade Center and the Pentagon demonstrated the resolve of average Americans.

"We refuse to bend, refuse to change, refuse to yield to fear," Biden said. "That is what makes us so proud of this city and this state, what makes me be so proud to be an American. It's that we have never, ever yielded to fear. Never."

At Tuesday's ceremony, which also featured performances by the Boston Pops and a youth choir, Roxbury Presbyterian Church Rev. Liz Walker recalled the three killed in the bombing, as well as Massachusetts Institute of Technology police officer Sean Collier.

"Today we remember Krystle Campbell, her energy and zest, her adventure and passion, a generosity of spirit, a light that will never fade," Walker said. "We remember Lingzi Lu, heart and sparking eyes, music and guilelessness, a welcome smile that beams forever ... We remember Martin Richard, tough and competitive, kind and caring, a Dorchester kid through and through. And we will remember Sean Collier, dedicated, with honor, trusted and respected."

Before the ceremony, Mayor Martin Walsh and Roman Catholic Cardinal Sean O'Malley, accompanied by the families of those killed in the bombing, began the day on a quiet note, visiting wreaths lain at the spots on Boylston Street where the bombs went off.

This year's Boston Marathon will take place under heightened security on April 21 with the 36,000 runners and tens of thousands of expected spectators facing new restrictions, including bans on carrying backpacks into the race corridor.

Boston police had to seal off the area near the finish line on Boylston Street late on Tuesday after two suspicious backpacks were found. They said they had taken a male suspect into custody.

The suspect, 25-year-old Kevin Edson of Boston, was walking in the area barefoot when he was stopped and questioned by police, the Boston Globe reported. He was detained after he said he had a rice cooker in his backpack, the newspaper said.

Police were still investigating the second backpack but it was unclear who the owner was. A bomb squad was called to dispose of both bags in controlled detonations, the newspaper reported.

Local media video footage online showed a barefoot man wearing a black hat and black paint on his face, carrying a backpack and yelling "Boston Strong" in the street.

Edson is to be arraigned on Wednesday on charges of possession of a hoax device and threat to commit a crime, among other charges, the newspaper said.

Authorities say the ethnic Chechen brothers, Dzhokhar and Tamerlan Tsarnaev, carried their bombs to the finish line in backpacks in last year's attack.

Three days later, the FBI released pictures of the suspected bombers and asked for the public's help in finding them. That prompted the brothers to try to flee Boston, which began with them shooting MIT police officer Collier, prosecutors said.

The resulting police chase ended in a gunbattle in the Boston suburb of Watertown. Tamerlan, 26, was killed; Dzhokhar, now 20, escaped before being captured on April 19.

The surviving brother is awaiting trial on charges that carry the possibility of execution if he is convicted.

(Editing by Paul Thomasch, Jonathan Oatis, Ken Wills and Sonya Hepinstall)