Messages of sympathy and solidarity poured in from nations, particularly those whose civilian populations have been targeted by previous attacks.
"We Spaniards know well the suffering that the British people are going through today," Prime Minister Jose Luis Rodriguez Zapatero said, referring to train bombs in Madrid last year that killed 191 people.
US President George Bush stood side by side with British Prime Minister Tony Blair at the G8 summit meeting in Gleneagles, Scotland, to say world leaders would react resolutely.
"Their resolve is as strong as my resolve," Bush said. "We will find [the perpetrators]. We will bring them to justice. And at the same time we will spread an ideology of hope and compassion that will overwhelm their ideology of hate."
Australia expressed worries at a lack of intelligence on the bombings. "To the best of my knowledge there wasn't any intelligence ... that is of some concern," said Foreign Minister Alexander Downer.
There was solidarity for the victims, the British government and its citizens from statesmen, religious leaders and ordinary people from round the globe.
"I grieve with all Londoners at the wounds that have been inflicted on this wonderful city - the city that is home to people from so many countries and cultures," said UN Secretary-General Kofi Annan.
UN chief Kofi Annan said he
grieved with all Londoners
Russian President Vladimir Putin said the bombings were "inhuman crimes", Irish Prime Minister Bertie Ahern called them "a black mark on society" and Pope Benedict XVI deplored "these barbaric acts against humanity".
Indonesia, the world's most populous Muslim nation, said the attacks showed no country was immune from terrorism.
Joy into sorrow
The state China Daily newspaper said in an editorial: "We are in deep sorrow for the cataclysm that has been visited upon the city of London, just one day after we extended our congratulations on its winning the 2012 Summer Olympics."
The International Olympic Committee expressed grief. "I'm deeply saddened that this should happen at the heart of an Olympic city," IOC president Jacques Rogge said.
"Unfortunately there is no safe haven. No one can say their city is safe," he added.
"Unfortunately there is no safe haven. No one can say their city is safe"
Jacques Rogge, International Olympic Committee president
Israeli Foreign Minister Silvan Shalom sought to compare the attacks with his country's struggle against Palestinian fighters.
"This attack shows us once again that terrorism is not Israel's problem only," he said.
The Palestinian resistance group Hamas, responsible for many attacks on Israelis, condemned the London bombings.
"Targeting civilians in their transport means and lives is denounced and rejected," said Moussa Abu Marzouk, deputy chief of the group's political bureau.