The British public, meanwhile, planned to thwart cosy images of the transatlantic alliance as anti-war protestors prepared to descend onto the streets of London by the tens of thousands.
Bush's entourage was speeding toward London's Westminster Abbey for a solemn wreath-laying ceremony as the first pictures of bodies and mangled wreckage on the streets of Istanbul appeared on TV.
The strikes hit targets including Britain's consulate and the office of its largest bank, HSBC.
Despite dark skies and drizzle, protest organisers were expecting 100,000 demonstrators to march past Downing Street - where Bush and Blair will meet - and into Trafalgar Square.
There, in what the Stop the War Coalition hopes will be the most poignant image of dissent over the Bush visit, they will topple a papier mache effigy of the US leader in a mock recreation of the way victorious US troops pulled down a Saddam Hussein statue in Baghdad in April.
"There were 25,000 people demonstrating in various cities around the country last (Wednesday) night so we expect a much bigger turnout today," Stop the War convener Lindsey German told BBC radio.
"We are expecting it to be totally peaceful," she added.
More than 5000 police will be on duty for the march, as an unprecedented security operation remains in force to protect Bush and his 700-strong entourage from the twin fears of unruly protest and a terrorist incident.
Bush arrived Tuesday for the first state visit ever by a US leader to Britain, staying at Buckingham Palace as the personal guest of Queen Elizabeth II who Wednesday treated him to a ceremonial welcome and lavish state dinner.
He also gave one of the most important foreign policy speeches of his career, in which he set out what he called his "three pillars" for global security - including the need at times to go to war to safeguard freedom.
His talks with Blair on Thursday were certain to focus on the latest blasts in Istanbul, as well as Iraq, including prospects of a transfer of sovereignty in Baghdad next June while fighting against guerrillas targeting US, British and other occupation forces.
Police monitor the security operation around Bush's visit
Bush and Blair were also to discuss the Middle East, Iran's nuclear program, and ways to mend EU-US relations as a trade war looms over US tariffs on imported steel - tariffs ruled illegal by the World Trade Organisation.
An announcement could also be made on the fate of nine British detainees held without trial at a US military base in Guantanamo Bay, Cuba, and regarded by Washington as enemy combatants ineligible for civil trials.
Several hundred demonstrators massed at the front gates of Buckingham Palace at dusk on Wednesday to jeer Bush and burn a US flag, in the most vivid act of protest so far in the visit. There were some scuffles, but no serious clashes.
Blair's government was more worried about a British newspaper's startling success - revealed Wednesday - in planting a reporter in the palace for two months posing as a footman, using bogus references. A probe is underway.
In his speech Wednesday, Bush - in what seemed to be a crack at France and Germany, which opposed his Iraq campaign - told the world's democracies to stop tolerating tyrants and join America in spreading freedom around the globe.
He defended his decision to wage war on Saddam Hussein's Iraq, saying "violent restraint of violent men" was sometimes a necessary option to confront repression.
The queen said Britons admired Bush's post-9/11 actions
But he also spoke of the need for "effective" multilateral institutions, and for the "global expansion of democracy" - not least in the Arab world.
"If the greater Middle East joins the democratic revolution that has reached much of the world, the lives of millions in that region will be bettered, and a trend of conflict and fear will be ended at its source," he said.
Bush, who paid an official visit to London in July 2001, was invited 17 months ago to return for a formal state visit.
During dinner on Wednesday, the queen paid tribute to the president, claiming to speak for her country.
"The leadership you showed in the aftermath of the terrible events of September 11, 2001 won the admiration of everyone in the United Kingdom," she told Bush in her toast.