Turkey and the UK have pledged not to bow to terror following the attacks on the UK consulate and a British bank in Istanbul that killed 27 people.
Turkish and UK police are investigating the blasts
An intensive investigation is under way into the suicide bombings, with 16 officers from London's anti-terrorist squad joining Turkish colleagues.
Turkey's foreign minister confirmed several arrests had been made but said it was too early to give more details.
Some foreign firms have begun sending their staff home.
Turkish Foreign Minister Abdullah Gul said the attacks would strengthen the global fight against terrorism.
Addressing a joint news conference, his UK counterpart, Jack Straw, expressed his government's support for Turkey.
The second blast devastated parts of the British consulate
The Foreign Office warned British citizens against travel not only to Istanbul, but to all of the country's other major cities, including the capital Ankara.
The US State Department has closed its consulate in Istanbul and warned its citizens to stay away form the area where the bombings took place.
The mass circulation Turkish daily Hurriyet has reported that seven people had been arrested and identified by police as Turkish citizens.
Without giving further details of the arrests, Mr Gul vowed that police "will definitely find out who is behind this attack and what their intention is."
Turkish and UK officials have already said the attacks bore the hallmarks of al-Qaeda and its associates.
Mr Straw said the suicide bombings in Istanbul - following similar attacks around the world - showed that the civilised world was facing a global threat which must be tackled in a global way.
Asked whether security at the British consulate had been tight enough, Mr Straw said whatever precautions were taken, some terrorists would always get through.
But he added: "We must never ever shift responsibility for the atrocities and the deaths away from the terrorists."
Earlier on Friday, Mr Straw toured the HSBC headquarters, the first building to be hit.
"This is an attack on civilisation," Mr Straw said as he picked his way through the rubble, describing the blasts as the work of "ruthless fanatics."
Mr Straw added that the bombings had increased the ties between his country and Turkey.
"Far from this hurting Turkey's application to join the European Union, it will increase the determination of all of us to see Turkey a full member of the European Union," he said.
The two blasts, which also injured more than 450 people, follow last Saturday's suicide bomb attacks against two synagogues in Istanbul that left 25 people dead.
A van is believed to have exploded near the HSBC building, devastating its facade.
Minutes later, another van crashed through the gates of the British consulate and blew up, killing 16 people, among them the UK Consul-General Roger Short.
The blasts coincided with a visit to the UK by President George W Bush, who said the bombers had shown "utter contempt" for human life.
An unconfirmed report said a group linked to al-Qaeda, calling itself Abu Hafz al-Masri Brigades, had issued a statement claiming responsibility for the blasts.
A man had earlier called the semi-official Anatolia news agency in Turkey to say al-Qaeda and the Turkish Islamic militant group IBDA-C had jointly carried out the attacks.
WHO ARE AL-MASRI BRIGADES?
Named after Bin Laden's brother-in-law
Claimed responsibility for attack on UN in Iraq
Also said they were behind blasts in Kenya and Turkey
Statements impossible to verify, some commentators sceptical
Both groups had said they were behind the synagogue blasts.
In Washington, US Deputy Defence Secretary Paul Wolfowitz said this kind of attack would encourage unity between the US and its allies, rather than drive them apart.
"This kind of horrible act... bring Turkey and the United States and for that matter the United Kingdom and the entire civilised world closer together," Mr Wolfowitz said.
The Foreign Office in London issued the following number for people to call for information about those who may have been involved in the blasts: 020 7008 0000.