Pope Francis and Archbishop Justin Welby join world leaders reacting to the Paris attacks in prayer.
Justin Welby responded to the attacks, which began on Friday evening and is thought to have killed 128 people, with the words:
"Tragic Paris, desperate news of deep tragedy, with heart break for so many. We weep with those affected, pray for deliverance and justice."
Pope Francis condemned "this new manifestation of maddening, terrorist violence and hatred." He deemed them unjustifiable "inhuman" acts that left him shaken and pained.
There is no justification for these things," he said in an extremely sombre voice in a telephone call to an Italian Catholic television station. "This is not human," he said, adding that he was praying for the victims, their families and all the French people.
A statement from the Vatican released on Friday declared the attacks "an attack on peace for all humanity" and called for "a decisive, supportive response on the part of all of us as we counter the spread of homicidal hatred in all its forms."
David Cameron said: "I am shocked by events in Paris tonight.
"Our thoughts and prayers are with the French people. We will do whatever we can to help."
The US president told a press conference: "Those who think they can terrorise the people of France and the values they stand for are wrong."
Labour leader Jeremy Corbyn condemned the attacks as "heinous and immoral."
"My thoughts are with the people of Paris tonight," the Labour leader said. We stand in solidarity with the French."