The Group of Eight most industrialised nations declared at their summit in Gleneagles, Scotland, that the bombings were attacks on the civilised world and said they would stand together to defeat militants.
US President George W Bush vowed, in a separate statement to reporters in Gleneagles, that the war on terrorism would continue until “an ideology of hate” had been overcome.
“They have such evil in their heart that they will take the lives of innocent folks. The war on terrorism is on,” Mr Bush said.
The UN Security Council unanimously passed a resolution condemning the bombings, saying it regarded any act of terrorism as “a threat to peace and security” as Secretary General Kofi Annan expressed solidarity with Britain.
“Today, the world stands shoulder to shoulder with the British people, who with others around the world had mobilised so powerfully against poverty and climate change ahead of the Group of Eight summit, and who, I am sure, will confront this ordeal with the same spirit, courage and determination,” he said.
Iraqi President Jalal Talabani said the explosions that hit London proved that terrorism was a global plague that could strike anywhere.
“What is happening in Iraq can happen in any country,” Mr Talabani said.
Australian Prime Minister John Howard, whose country has troops in Iraq, said such attacks would not alter “the determination of free countries to do the right thing”.
Spain, recalling the Madrid train bombings in March 2004, which killed 191, strongly condemned the London blasts.
“We, Spanish people, know the suffering British people are going through today,” Prime Minister Jose Luis Rodriguez Zapatero said.
French President Jacques Chirac, who has frequently clashed with Mr Blair of late over EU disputes, pledged his country’s “total solidarity”.
German Chancellor Gerhard Schroeder, who had also recently sparred with the British leader, threw his weight behind Mr Blair.
“We agree that the international community must do everything in its power to fight terrorism together with all the means at its disposal,” he said.
The Vatican’s secretary of state, Cardinal Angelo Sodano, called for “men of goodwill and all religions” to bring to an end to the “clash of civilisations.”
“In the name of our Heavenly Father we must bring this clash of civilisations to an end, humanity must embark on a new era,” Cardinal Sodano told Italian television TG5.
European Union commission head Jose Manuel Barroso called the blasts an “attack on democracy” and the “fundamental freedoms” that are at the heart of the 25-nation bloc.
As condolences poured in from around the globe, NATO Secretary General Jaap de Hoop Scheffer said the alliance would not waver in its struggle against violent extremists.
Russian President Vladimir Putin said the horrifying images from London exposed rifts in the global war on terror and urged an end to what he called “double standards” on security.
China said random violence against civilians was never justified, while Japan, which was hit by subway attacks a decade ago, offered its full support to Britain as Prime Minister Junichiro Koizumi expressed his “furious anger.”
Kenya, which has been twice hit by al-Qaeda attacks, expressed sympathy with the victims in London, saying they proved terrorism was a global problem from which no country is immune.
Israeli Foreign Minister Silvan Shalom said he hoped the blasts would increase vigilance against extremists.
Nigerian President Olusegun Obasanjo of Nigeria, who was in London when the bombs struck, expressed solidarity in a letter to Mr Blair, who he noted was campaigning at the G8 summit to relieve poverty in Africa.
Pakistan, which has a one-million-strong community in Britain, also denounced the assault.
Iran “condemned the terrorist attacks that caused deaths and injuries
among British citizens” and rejected the use of violence for political ends.
World financial leaders meanwhile appealed for calm after markets were left badly shaken by the blasts in London, Europe’s foremost trading centre.
European stock markets plunged, sterling slid against major currencies and oil prices plummeted from record peaks reached yesterday owing to concerns that the attacks would dent global growth and discourage airline travel.
On Wall Street both the Dow Jones and Nasdaq averages regained positive territory after heading down earlier.