Police captured a 19-year-old ethnic Chechen suspected of carrying out the Boston Marathon bombings with his older brother after an intense day-long manhunt on Friday that closed down the city and turned a working-class suburb into a virtual armed camp.

The break in the case sent waves of relief and jubilation through Boston and the suburb of Watertown, where armoured vehicles roamed the streets and helicopters flew overhead through the day. Residents and police officers cheered and clapped when the suspect, Dzhokhar Tsarnaev, was caught after an exchange of gunfire with police.

A Massachusetts State Police spokesman said Tsarnaev was bleeding and in serious condition in a Boston hospital.

He had been hiding in the stern of a boat parked in the backyard of a house in Watertown, police said. A resident called police after seeing blood on the boat.

President Barack Obama told reporters at the White House after the suspect's capture that questions remained from the bombings, including whether the two suspects received any help.

Monday's bombing has been described by Obama as "an act of terrorism." It was the worst such attack on US soil since the plane hijackings of September 11, 2001, and set nerves on edge across the United States with a series of security scares.

The Boston Police Department said in a message on Twitter: "CAPTURED!!! The hunt is over. The search is done. The terror is over. And justice has won. Suspect in custody."

Tsarnaev is one of two brothers believed to have set off bombs made in pressure cookers and packed with ball bearings and nails at the finish line of the world-famous event, killing three people and injuring 176. The family of Martin Richard, an 8-year-old boy killed in the blast, cheered his capture.

"Tonight, our community is once again safe from these two men," the family said in a statement.

The older brother, Tamerlan Tsarnaev, 26, was killed on Thursday night in a shootout with police less than a mile from where Friday night's capture took place. The FBI had publicised pictures of the two men on Thursday and asked the public for help in identifying them.

Hours after the photos were released, a furious sequence of events erupted, including the fatal shooting of a police officer on the campus of the Massachusetts Institute of Technology.

The night culminated in a firefight in Watertown during which police say the brothers threw bombs at officers. Tamerlan suffered fatal wounds, while the younger brother escaped on foot.

The brothers had not been under surveillance as possible militants, US government officials said. But the FBI said in a statement on Friday that in 2011 it interviewed Tamerlan at the request of a foreign government, which it did not identify.

"The request stated that it was based on information that he was a follower of radical Islam and a strong believer, and that he had changed drastically since 2010 as he prepared to leave the United States for travel to the country's region to join unspecified underground groups," the FBI statement said.

The matter was closed because interviews with Tamerlan and family members "did not find any terrorism activity, domestic or foreign."

At the same time that police were pursuing Dzhokhar on Friday night, police in New Bedford, Massachusetts, 96 km south of Boston said three other people had been taken into custody for questioning about Monday's bombings. They were later released, police said.

People in Boston reacted mostly with glee to the news of Dzhokhar's capture. On a warm evening, people came out of their homes in and around Kenmore Square, whooping and cheering and simply running together in large groups, holding hands. Some waved US flags.

Student Rita Pratte, 19, said, however that she did not feel at rest. "It's just shaken me up. Looking around I feel suspicious of more people than I ever did before. I just don't feel like it's settled."

After the capture of Dzhokhar Tsarnaev, authorities said the investigation was still open. They were asked at a news conference on Friday night why authorities did not immediately read the suspect a warning usually given by police to criminal suspects in custody before they are interrogated so their statements can be admissible in court.

Massachusetts US Attorney Carmen Ortiz said, "There is a public safety exception" that was applied to Tsarnaev regarding the warning known as Miranda rights.

Earlier on Friday, officers went door-to-door in Watertown and searched houses. During the search for the men on Friday, two Black Hawk helicopters circled the area. SWAT teams moved through in formation.

The normally traffic-clogged streets of Boston were empty on Friday as the city went into lockdown during the manhunt. Public transportation had been suspended and air space restricted.

Famous universities, including Harvard and MIT, closed after police ordered residents to remain at home.

Massachusetts Governor Deval Patrick lifted the "stay-in-place" order for Boston late on Friday afternoon.

'PUT A SHAME ON OUR FAMILY'

Some details emerged on Friday about the brothers, including their origins in the predominantly Muslim regions of Russia's Caucasus, which have experienced two decades of violence since the fall of the Soviet Union.

The younger brother described himself on a social network as a minority from a region that includes Chechnya, Dagestan and Ingushetia.

A man who told reporters he was an uncle of the brothers said they came to the United States in the early 2000s and settled in the Cambridge, Massachusetts, area.

Ruslan Tsarni, who lives in suburban Washington and has not spoken to the brothers since 2009, said the bombings "put a shame on our family. It put a shame on the entire Chechen ethnicity."

In separate interviews, the parents of the Tsarnaev brothers said they believed their sons were incapable of carrying out the bombings. Others remembered the brothers as friendly and respectful youths who never stood out or caused alarm.

"Somebody clearly framed them. I don't know who exactly framed them, but they did. They framed them. And they were so cowardly that they shot the boy dead," father Anzor Tsarnaev said in an interview with Reuters in Dagestan's provincial capital, Makhachkala, clasping his head in despair.

The FBI said the twin blasts were caused by bombs in pressure cookers and carried in backpacks that were left near the marathon finish line as thousands of spectators gathered.

The mother, Zubeidat Tsaraeva, speaking in English, told CNN, "It's impossible, impossible, for both of them to do such things, so I am really, really, really telling that this is a setup."

The bombings elicited a response from Moscow condemning terrorism and from the Russian-installed leader of Chechnya, who criticised police in Boston for killing an ethnic Chechen and blamed the violence on his upbringing in the United States.

The Islamic Society of Boston Cultural Center, the biggest mosque in the area, said in a statement that it was shutting its doors until further notice. – Reuters