World reaction: Political and religious leaders from across the globe, from Pope Benedict to the heads of Palestinian Islamic group Hamas, condemned the terrorist attacks in London.
The UN, EU and Nato were among the organisations to voice their solidarity with the UK.
As well as expressing their condolences to the victims, many governments announced plans to step up security co-operation and other efforts aimed at combating terrorism.
Spain was one of the first countries to express revulsion at the attacks. Conscious of the parallels with Madrid's al-Qaeda-linked bombings of March 2004, the country's prime minister, José Luis Rodríguez Zapatero, immediately announced in a televised broadcast that he had put domestic security forces on full alert.
Calling for unity against "the barbarism of terrorism", Mr Zapatero said: "The Spanish government condemns with all its energy the brutal attacks today in London and expresses its deepest solidarity with the people of that city, their government, the authorities and the British people."
Also quick to respond to the attacks was Pope Benedict, who described them as "barbaric acts against humanity".
A telegram sent by the Vatican's secretary of state to Britain's Cardinal Cormac Murphy-O'Connor, archbishop of Westminster, described the Pope as "deeply saddened" by the news, adding that "the Holy Father offers fervent prayers for the victims and for all those who mourn.
"Upon the people of Great Britain he invokes the consolation that only God can give in such circumstances," the telegram continued.
Archbishop of Canterbury Dr Rowan Williams, who had been holding an interfaith meeting with Muslims when news of the blasts came through, said everyone caught up in the tragedy "are in my prayers and in the prayers of a great many people".
Irish and other MEPs meeting in Strasbourg yesterday responded to the news with a show of unity.
European Parliament president Josep Borrell said: "I want to send, on behalf of the parliament, a message of solidarity to the British people. We all stand with you today, and we will never let the atrocities of terrorism defeat the values of peace and democracy in Europe."
Dutch prime minister Jan Peter Balkenende announced that security was being stepped up at the Dutch borders and around British sites in the Netherlands.
Security was also stepped up in Hungary, where unrelated bomb threats led to the evacuation of three Budapest shopping centres.
News of the blasts brought an end to the party atmosphere in Singapore, where International Olympic Committee president Jacques Rogge said: "I'm deeply saddened that this should happen at the heart of an Olympic city. Unfortunately there is no safe haven. No one can say their city is safe."
Nato secretary general Jaap de Hoop Scheffer said the attacks "underline the need for the international community and members of the alliance to remain united in the fight against terrorism".
In the Middle East, spokesmen for the governments of Saudi Arabia, Lebanon, the United Arab Emirates and Morocco condemned the attacks, along with officials in the Palestinian authority.
Israeli foreign minister Silvan Shalom said: "This attack shows us once again that terrorism is not Israel's problem only."
Palestinian Islamist group Hamas, responsible for many suicide attacks on Israelis, also condemned the London bombings. "Targeting civilians in their transport means and lives is denounced and rejected," Moussa Abu Marzouk, deputy chief of the group's political bureau, said.
