The international community condemned the bombings at the Boston Marathon on Monday, which have killed three people and injured more than 100 and forced many U.S. cities to step up security.
UN Secretary-General Ban Ki-moon on Monday said the bombing "is all the more appalling for taking place at an event renowned for bringing people together from around the world in a spirit of sportsmanship and harmony."
Speaking at a ceremony at UN headquarters commemorating the 19th anniversary of the Rwanda genocide, Ban called the Boston Marathon explosions "senseless violence."
"For now I just wanted to say that my thoughts are with everyone in Boston," Ban said, expressing his deepest condolences to the families of the victims and wishing those wounded a speedy recovery.
Canadian Prime Minister Stephen Harper said he was shocked to learn of the explosions that occurred on Monday during the running of the Boston Marathon.
"It is truly a sad day when an event as inspiring as the Boston Marathon is clouded by such senseless violence," Harper said.
"Our thoughts and prayers are with those injured or affected by this horrible incident. We stand with our American neighbors in this difficult time," he said.
While some U.S. media and experts started to use the phrase of "terrorist bombings," President Barack Obama remained cautious and avoided calling the blasts a terror attack.
Obama said his government has not yet got "all answers" about the Boston explosions, but vowed to hold all those behind them accountable.
He also said the government has taken steps to beef up security across the country following the two blasts that happened at around 3 p.m. local time (1900 GMT) and went off within seconds of each other near the Boston Marathon finish line.
Deputy Police Commissioner of the New York Police Department (NYPD) Paul Browne told local media that the city was stepping up security at hotels and other landmark buildings with deployment of critical response vehicles.
Browne added that the additional security will remain until more is learned about the explosions.
Chicago police, transit agencies and emergency agencies have also beefed up security following the bombings.
According to Chicago Tribune, police are checking buildings and sites considered possible high-priority targets. The Office of Emergency Management and Communications is broadcasting a reminder to officers to follow proper procedures when investigating suspicious packages and persons.
The commuter train system Metra, the Chicago Transit Authority and other agencies are urging riders to notify officials if they see anything suspicious.
The police department and other first responders are monitoring events closely and communicating with law enforcement officials as a precautionary measure.
"At this time, we are not aware of any threats facing Chicago," police spokesman Adam Collins said.
In Los Angeles, officials said there would be an increased police presence at all scheduled sports events in the near future, including the deployment of bomb-squad personnel, dogs and other precautions geared to preventing a similar event.
Meanwhile, police in San Francisco said they were on heightened alert and urged people to call 911 if they see anything suspicious.
