Countries and international organizations, including the United Nations, China, Germany, France, Ukraine and Russia condemned the bombings and extended condolences to the victims.

UN Secretary-General Ban Ki-moon on Monday said the attack "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."

Chinese Foreign Ministry spokeswoman Hua Chunying told a press briefing Tuesday that China strongly condemned and is firmly opposed to any attack aimed at common people.

"We extend our deep condolences and sincere regards to the victims and their relatives," she said.

"Nothing justifies such a vicious attack on people who had gathered for a peaceful sporting event," German Chancellor Angela Merkel said.

The French Foreign Ministry said in a statement that "our thoughts are with the victims' families and the injured."

"We also want to assure the local and federal authorities of our support and our wholehearted solidarity," the ministry said.

Ukrainian Foreign Minister Leonid Kozhara on Tuesday condemned the bombings and called for strengthened coordination among different nations to fight with the global threat of terrorism.

"The terrorist attack in Boston indicates that we must unite our efforts," Kozhara said.

"Russian President Vladimir Putin strongly condemned that barbaric crime and expressed his belief that the combat against terrorism requires active coordination of the efforts of the entire international community," the Kremlin press service said Tuesday, adding that Moscow is ready to assist Washington in investigating the bombings.

Russian Prime Minister Dmitry Medvedev called the bombing attack "a terrible crime."

The Madrid Marathon said it will strengthen its security measures as result of the Boston bombings, while Russia will tighten security for this summer's world athletics championships.

Meanwhile, people in Houston, Texas, gathered for an evening vigil to remember the victims of the bombings.

Houston City Council member Helena Brown, organizer of the vigil in front of City Hall, called the bombings "a tragic attack on innocent victims."

Politicians elsewhere in Texas also expressed support for the victims and their families.

"The scene at the Boston Marathon today is a sobering vision for us all, especially those who have friends or loved ones competing in today's race," Texas Governor Rick Perry said in a statement.