Barack Obama has led world condemnation of the terror attacks in Paris, calling them an "outrageous attempt to terrorise innocent civilians" and vowing to do whatever it takes to help bring the perpetrators to justice.
Speaking to reporters at the White House, the US president said he would not speculate about who was responsible.
He called the attacks a "heartbreaking situation" and an "attack on all of humanity".
UN secretary-general Ban Ki-moon hit out at "the despicable terrorist attacks".
Spokesman Stephane Dujarric said Mr Ban "trusts that the French authorities will do all in their power to bring the perpetrators to justice quickly".
The UN Security Council also condemned "the barbaric and cowardly terrorist attacks", and underlined the need to bring the perpetrators of "these terrorist acts to justice".
Angela Merkel said she was "deeply shaken by the news and pictures that are reaching us from Paris".
The German chancellor issued a statement saying her thoughts were with the victims "of the apparent terrorist attack".
The Vatican also condemned the assault as "an attack on peace for all humanity".
Spokesman the Rev Federico Lombardi said in a statement that the violence requires "a decisive, supportive response on the part of all of us as we counter the spread of homicidal hatred in all of its forms".
Israeli prime minister Benjamin Netanyahu said in a statement: "Israel stands shoulder-to-shoulder with French president Francois Hollande and with the people of France in our common battle against terrorism."
In the Middle East, Iranian president Hassan Rouhani sent a message to Mr Hollande condemning the terror attacks.
The state-run IRNA news agency quoted Mr Rouhani as saying that Iran "itself has been a victim of the scourge of terrorism" and the fight must go on.
Mr Rouhani cancelled visits to France and Italy, due in a few days. France was one of the world powers involved in recent negotiations with the Islamic republic over its contested nuclear programme.
Hossein Jaber Ansari, a Foreign Ministry spokesman, was quoted by IRNA as saying: "Those terrorist groups that committed the Paris crimes do not believe in ethical principles and they are not loyal to any type of divine religions - including Islam."
Egyptian president Abdel-Fattah el-Sissi conveyed condolences to the victims.
A statement from his office called for "concerted international efforts" to combat "the scourge of terrorism, which aims to destabilise security and stability in various parts of the world, without distinction".
Jordan's King Abdullah II "expressed strong condemnation and indignation at the cowardly terrorist act", and solidarity with the French people, in a statement published by state news agency Petra.
In the United Arab Emirates, the state-run WAM news agency said president Sheikh Khalifa bin Zayed Al Nahyan sent a telegram to Mr Hollande offering his condolences and pledging support for France. WAM said Mr al Nahyan also supported doing "what it takes to face terrorism and eliminate it".
The ruler of Kuwait, emir Sheikh Sabah Al Ahmad Al Sabah, said in a statement that he offered his condolences, while stressing that "these criminal acts of terrorism ... run counter to all teachings of holy faith and humanitarian values".
In Saudi Arabia, the state-run Saudi Press Agency quoted a Foreign Ministry official denouncing the attack.
US secretary of state John Kerry described the attacks as "heinous, evil and vile", calling them "an assault on our common humanity".
Mr Kerry said the US embassy in Paris is "making every effort to account for the welfare of American citizens in the city", and the US stands ready "to provide whatever support the French government may require".
US defence secretary Ash Carter called the attacks "an assault on our common human dignity".
The Pentagon chief said: "The United States stands with the people of France and its vibrant, multicultural democracy."
He praised France as a Nato ally and a leader of the coalition fighting Islamic State militants in Syria.
Canadian prime minister Justin Trudeau said: "Our hearts and thoughts and prayers go out to our French cousins in this dark and terrible time."
He said Canada had offered "all of our help and support to the government of France".
China's Foreign Ministry spokesman Hong Lei says Beijing was "deeply shocked" by the attacks and pledged solidarity with France in combating terrorism.
"Terrorism is a common challenge facing humanity. China resolutely supports France in maintaining its national security and stability and in attacking terrorism," Mr Hong said.
Japan's foreign minister Fumio Kishida says he was "deeply shocked and outraged" by the news.
Speaking to reporters in Hiroshima, he said Japan stands by France, promising to co-operate in the international fight against terrorism.
"We strongly condemn the act of terrorism, which we do not tolerate for any reason," he said, expressing condolences to the victims and their families.
