ISTANBUL, Nov 21 (Reuters) - Turkey said on Friday police had arrested several people over twin truck bomb attacks on British targets in Istanbul that killed 27 people, including Britain´s top diplomat in the city.

A statement purporting to come from a unit of Osama bin Laden´s al Qaeda network said it carried out Thursday´s strikes on the British consulate and the London-based HSBC bank, five days after two similar attacks on Istanbul synagogues.

U.S President George W. Bush, on a state visit to Britain, pledged his solidarity with Turkey, a key NATO ally long promoted by Washington as a model of Islamic democracy.

Headlines in Turkish newspapers reflected the sense of shock in a nation still reeling from Saturday´s explosions.

"Al Qaeda is at war with Turkey," declared the Radikal newspaper as Istanbul awoke, shrouded in thick fog. Many Turks were killed or wounded alongside British citizens.

Photographs showed dazed and bloodied survivors at the scene of the blasts and the shattered HSBC and consular buildings.

Thursday´s twin bombings killed 27 people, including British Consul General Roger Short, and wounded more than 400.

On Saturday two Istanbul synagogues were devastated by suicide truck bombs that killed 25 and wounded 300, making this the worst week of peacetime violence in Turkey´s modern history.

Britain has warned more terror attacks may target Turkey.

"Some people have been arrested, but it´s too early to give any information about them," Turkish Foreign Minister Abdullah Gul told a news conference, referring to the Thursday blasts.

The Turkish daily Hurriyet said seven people had been arrested over the Thursday attacks, which it said were carried out by Turkish suicide bombers. A spokesman at the Istanbul governor´s incident room could not confirm the report.

DEFIANCE

HSBC Holdings Plc (HSBA.L: Quote, Profile, Research) , Britain´s biggest company, said it had resumed service in Turkey on Friday despite the blasts. HSBC Bank has about 160 branches in the country.

Other expatriates also sounded a note of defiance.

"This is not a September 11 attack. This can happen in Nigeria, America or even Germany," said German Christian Johannes, 33, a partner in a firm working in the energy sector.

Istanbul, a vibrant commercial city of 12 million, was on alert for possible further attacks as Turkey heads into the annual Muslim Bayram holiday next week.

Turkey´s National Security Council, an advisory body grouping political leaders and influential military commanders, was certain to focus on the attacks and counter-measures at a Friday afternoon meeting in the capital Ankara.

In a telephone conversation, Bush and Turkish Prime Minister Tayyip Erdogan "affirmed that they stand shoulder-to-shoulder in the fight against terrorism", the White House said.

British Foreign Secretary Jack Straw appeared with Gul at the Istanbul news conference, pledging support.

"This was as much an attack on the Turkish people as it was upon British interests, and we stand together with Turkey."

A statement apparently from a unit of Osama bin Laden´s al Qaeda network, the Abu Hafz al-Masri Brigades, and published on the Islamist Web site Al Mujahidoun, said it had carried out the latest attacks. It could not be independently authenticated.

The Abu Hafz claim, in Arabic, said Turkey was targeted because of its membership in the "crusader" NATO alliance and its ties with the "Zionist entity" Israel.

A Turkish group called the Islamic Great Eastern Raiders Front (IBDA- C) has also claimed joint responsibility with al Qaeda for all four Istanbul attacks and warned the network was planning more strikes against the United States and its allies.

Thursday´s double strike shocked world markets, already on edge after al Qaeda´s warning earlier this week of more attacks.

Security fears weighed on the European stock market mood on Friday, although the dollar regained ground.

Istanbul´s stock market, which shed 7.37 percent on Thursday before trade was halted, remained shut on Friday. Turkish debt prices slipped and the lira wobbled. (Additional reporting by Claudia Parsons in Istanbul) (© Reuters 2003 11/21/03)