Islamist terrorists were blamed for the London blasts by Prime Minister Blair last night. He insisted that Britain would not be “cowed” by the worst-ever terrorist atrocities in the capital.

In a televised statement made at Downing Street after the prime minister flew back to London from the Gleneagles summit, he promised that there would be “the most intense police and security service action to make sure that we bring those responsible to justice.”

Mr. Blair described the carnage in London as “a most terrible and tragic atrocity that has cost many innocent lives.”

The terrorists were trying to “frighten” and “cow” the public. But Britain would not be intimidated and Mr. Blair praised the “stoicism and resilience” shown by the people of London.

Paris stood shoulder to shoulder with London yesterday as Anglo-French rivalry gave way to unity in the face of terrorism.

“Right now, we are all Londoners,” the mayor of Paris, Bertrand Delanoe, said, one day after his city lost out to the British capital in its bid to host the 2012 Olympics.

Mr. Delanoe, who had complained of London’s unsporting behavior in its bid to hold the 2012 Games, dismissed Paris’s Olympic disappointment as insignificant compared to London’s ordeal.

“Life has been endangered in a friendly city,” he said. “The competition is derisory compared to the blasts.”

The bombings prompted sympathy and condolences from around the world, with countries affected by terrorism often reacting fastest.

In Middle Eastern capitals, some all too familiar with bloodshed, leaders said the attacks underscored terrorism’s global threat. President Talabani of Iraq, whose country is the scene of daily bombings, said: “What is happening in Iraq can happen in any country. I’m telling my Arab brothers that terrorism today in Iraq will tomorrow affect other Arab countries.”

The Israeli foreign minister, Silvan Shalom, linked events in London with his country’s struggle against Palestinian Arab militants who have used suicide bombings against civilians.

“This attack shows us once again that terrorism is not Israel’s problem only. Terrorism can hit everywhere and against everyone,” he said.

Palestinian Arab negotiator Saeb Erekat said: “We condemn with the strongest possible terms these explosions and convey our sincere condolences to the British people and government.”

Even nations and groups associated with terrorism joined the chorus of commiseration.

Syria’s president, Bashar al-Assad, condemned “these detested acts,” while the Palestinian Islamist group Hamas, responsible for dozens of suicide attacks on Israel, condemned the bombings, in an attempt to distance itself from Al Qaeda.

Arab satellite channels such as Al Jazeera and Al Arabiya aired live footage of the scenes in London, as did Lebanese and Israeli networks.

In his statement, Mr. Blair welcomed support from the Muslim Council of Great Britain condemning the terrorist acts.

He said: “We know that these people act in the name of Islam but we also know that the vast and overwhelming majority of Muslims here and abroad are decent and law-abiding people who abhor those who do this every bit as much as we do.”

Although Downing Street refused to state on the record that Islamist terrorists were responsible, a spokesman said Mr. Blair had “chosen his words carefully.”

Mr. Blair said Britain would show by its spirit and dignity that its values would long outlast those of the terrorists. “The purpose of terrorism is just that. It is to terrorize people and we will not be terrorized. This is a very sad day for the British people but we will hold true to the British way of life.”

The terrorists were trying to use the slaughter of innocent people “to cow us, to frighten us out of doing the things that we want to do, trying to stop us from going about our business, and they should not and must not succeed.

“When they try to intimidate us, we will not be intimidated. When they seek to change our country or our way of life by these methods, we will not be changed.”

Downing Street said Mr. Blair believed that it was important to show that world leaders would not be diverted from the work at the G8 summit.

The spokesman said: “This is not going to stop us doing the deal we want to do on climate change and the deal we want on Africa. We still believe this can be and it will be, on its own terms, a successful summit.”