President Jacques Chirac said France "learned with immense emotion of the monstrous attacks - there is no other word - that have just struck the United States." Mr Chirac broke off a visit to Brittany to return to Paris when he heard the news.
In what the French leader called "these terrifying circumstances", he said the French people "in their entirety stand beside the American people. They express their friendship, their solidarity in this tragedy. I naturally assure President George Bush of my total support."
Mr Chirac added that France "has always condemned terrorism without reservation, and considers that we must use all means at our disposal to fight against terrorism." The Prime Minister, Mr Lionel Jospin, telephoned the US ambassador to express his "horrified sadness" and called an emergency meeting with his ministers of foreign affairs, defence and the interior.
The defence ministry said it was stepping up French air defences, and the interior ministry put in place an "intensified" version of France's "Plan Vigipirate" - used during the 1998 World Cup to counter threats from Algerian extremists. The foreign ministry established a "crisis cell" and a dedicated telephone line for French citizens with relatives in Manhattan.
The French Foreign Minister, Mr Hubert VΘedrine, said he was "absolutely horrified and shocked by this wave of abominable attacks." France feels "total solidarity with the American people in this terrible trial," he added.
The German Chancellor, Mr Gerhard Schr÷der, called the attacks a "declaration of war against the entire civilised world". Whoever supported this act of terror has contravened the laws of the western world, he said. "I want to express to you and the US people my heartfelt sympathy and my unlimited solidarity," he said in a message to Mr Bush.
He conveyed his "abhorrence" at the attacks and said the German government condemned whole-heartedly "these terrorist acts".
The Interior Minister, Mr Otto Schily, ordered all government buildings to fly their flags at half-mast out of respect for the dead.