At least 26 people were killed and more than 400 injured in bombings of the British consulate and offices of the HSBC financial group in Istanbul today. Among those killed was the British Consul-General Roger Short.
Witnesses said suicide bombers used pickup trucks in the blasts that occurred five minutes apart, at about 11am.
In London, Foreign Secretary Jack Straw described the attacks as "clearly appalling acts of terrorism" adding: "I'm afraid it has all the hallmarks of international terrorism practiced by al-Qa'ida."
A call to the Anatolia news agency claimed that al-Qa'ida and the militant Islamic Great Eastern Raiders' Front, or IBDA-C, were responsible for the attacks.
Turkish authorities said the same groups were behind Saturday's nearly simultaneous synagogue bombings in Istanbul, which killed 23 people and the two attackers.
"It seems the attacks have been conducted with the same barbaric methods," Justice Minister Cemil Cicek, who serves as government spokesman, told reporters.
The first blast was at the Turkish headquarters of HSBC, the world's second-largest bank, sheering off the facade of the 18-floor building.
Body parts, the burned out shells of cars and broken glass were scattered through the streets outside the bank and bloodied bystanders looked dazed as they walked past lines of ambulances. Several people helped carry the limp bodies of victims.
Another bomb ripped off the wall surrounding the garden of the British consulate.
Mr Straw said three or four British employees from the consulate had not reported to a roll call following the blasts.
One witness was travelling on the bus near the bank when the explosion occurred.
"I thought somebody hit our bus from the back, then I saw black smoke rising. Cars were damaged all around us. I saw the charred body of a driver at the wheel," said a sobbing Mehmet Altan.
"After the blast the bus doors got stuck and passengers broke the windows to get out. There were pieces of flesh spread all around," bus driver Necati Erkek said.
Another witness, Hakan Kozan, aged 29, who was close to the British consulate at the time of the explosion, said a white pickup truck was responsible for the blast.
"I heard a slam on the brakes and 10 seconds later the explosion came," he told The Associated Press.
Mehmet Celik, who was slightly injured in the attack, said a light brown pickup truck "exploded in front of the HSBC headquarters."
Trading on the Turkish stock market was suspended. Some businesses, including the leading Yapi Kredi bank near HSBC and an IBM office near the British consulate, halted operations after the explosions.
The British consulate is located in the cramped historic Beyoglu district, a popular tourist destination with shops, bars, cinemas and restaurants.
The nearby US consulate was moved months ago to a new, more secure location in another district.
Turkey's Interior Minister Abdulkadir Aksu said the attacks targeted British-related institutions and appeared to be linked to Saturday's synagogue suicide bombings.
Yesterday, authorities arrested six people in connection with the synagogue bombings. A Turkish court charged five with "attempting to overthrow the constitutional structure," which carries a sentence of life imprisonment. A sixth person was charged with "helping illegal organizations," punishable by five years in prison.
Two suicide attackers, both Turks, blew up pickup trucks outside the synagogues on the Jewish Sabbath, killing 23 people and the two bombers. Foreign Minister Abdullah Gul said the two had visited Afghanistan in the past and that investigators were looking for any al-Qa'ida links.
On Sunday, al-Qa'ida claimed responsibility for the bombings in messages to two Arabic-language newspapers, but it was not possible to authenticate those claims. The outlawed IBDA-C also claimed responsibility, but Turkish authorities said the attack was too sophisticated to be carried out by that group.