President Obama, speaking from the White House, called the bombings in Boston “an act of terrorism,” following a briefing by his national security team.

“This was a heinous and cowardly act. Given what we now know about what took place, the FBI is investigating it as an act of terrorism,” Obama said. “Any time bombs are used to target innocent civilians, it is an act of terror. What we don’t yet know however, is who carried out this attack or why, whether it was planned and executed by a terrorist organization, foreign or domestic, or whether it was the act of a malevolent individual.”

Obama said it will take time to follow every lead and determine what happened, but, “We will find whoever harmed our citizens and we will bring them to justice. We also know this — the American people refuse to be terrorized.”

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With scores of FBI and ATF agents scouring the crime scene, the Boston Police Department said Tuesday that the area near the two explosions will remain closed for the foreseeable future. While the FBI said there is no sign of an imminent threat, Boylston Street remains sealed off from Berkeley Street to Mass Ave. and city and state officials were still encouraging Back Bay residents to stay at home and away from public spaces to allow investigators to do their jobs. There is said to be a good number of backpacks and bags to examine due to the fact that many marathon spectators who fled the finish line had been carrying them with extra clothing for the runners they were cheering on.

Those who did venture out Tuesday saw downtown streets free from any traffic and reams of police officers, bomb squads and ATF officials patroling the area. Military Humvees and police cars lined Boston Common, the oldest city park in the U.S. The 50-acre green is also where runners were funneled after Monday’s incident and many nearby residents poured into the streets to give marathoners clothing and food from their homes.

Marathon Sports, a seven-unit specialty retailer which has a store located next door to one of the explosions, encouraged visitors to its Web site to carry on. The company’s Twitter feed was posted on its home page with the lead item being, “Surreal morning. Know this: Boston is strong, and the running community is unified. We will persevere, prosper and we will run again.”

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The Prudential Center, which like Copley Place, is as much as a pedestrian walkway as it is a shopping center, has reopened but it can only be accessed by the Huntington Avenue side. A Prudential Center spokesman declined to say what percentage of the stores were open, noting that it was left to the tenants’ discretion. Copley Place’s 75 stores had the option to remain closed, according to a spokesman for its parent company Simon Properties Group. “With the opening of area hotels, Huntington Avenue and Stuart Street, and with the full operation of the Back Bay T station and the Prudential and Hynes T stations, we have informed the public as well as office and retail tenants that Copley Place is open today, as our building serves as a pedestrian connection,” he said.

J. Crew reopened its three stores in Copley Place as well as its Madewell store on Newbury Street. Talbots planned to keep its Boylston Street store closed Tuesday.

Meanwhile in New York, where more than 10 million visit the city on any given day, according to Police Commissioner Ray Kelly, there was a noticeable increase in police presence. The pedestrian plaza in Times Square was cordoned off with police tape and police cruisers were parked with flashing lights Tuesday morning. Officers were stationed outside the Mandarin Oriental Hotel located adjacent to the Shops at Time Warner.