At least 50,000 people set off on a march that took almost two hours to clear its starting point at the University of London. They passed parliament and the prime minister's residence on their way to Trafalgar Square where several thousand more protesters gathered ahead of the march.
Metropolitan police and march organizers have worked closely together and so far that's made for a very smooth protest, says CBS News Reporter Steve Holt.
The chief steward of the march, Chris Nineham, had predicted at least 100,000 people would join in, but as darkness fell, it appeared the numbers of protesters participating were far short of this prediction.
"I doubt we'll achieve much today but at least I'll have registered my protest," said Brother Oswin, who was wearing the brown robe and sandals of the Catholic Society of St. Francis.
Police said heavy security deployed for Wednesday's scattered protests against the president's state visit was in place for the march, which began hours after deadly bomb attacks on British targets in Istanbul, Turkey.
Deputy Assistant Commissioner Andy Trotter of London's Metropolitan Police said the Turkish attacks, which officials quickly blamed on al Qaeda, "underlines the need for such high-level security." He said more than 5,000 officers are on duty in the city.
Since the start of the state visit on Tuesday night, police said about 50 people had been arrested on charges that included theft, drunkenness and drug possession.
As marchers chanting "George Bush, terrorist" made their way through a business district, a few scuffled with three Bush supporters holding U.S. flags and a sign saying "support America." Police quickly intervened and bundled the trio into a nearby office building.
"I think it's a disgrace that these people are basically siding with Saddam Hussein and al Qaeda," said one of the three, Londoner Robert Temple. "Where were they when (former Romanian dictator Nicolae) Ceausescu came to town and why aren't they protesting against the people who blew up Turkey today?"
But some protesters said U.S.-British policy in Iraq was helping fuel terrorist attacks.
"It wouldn't have happened without Iraq. ... America is creating their own terrorists," said Ziggy Dlabal, a German sociologist who lives in London.
President Bush and Prime Minister Blair said they remained united against terrorism in the wake of the attacks in Turkey.
At a joint press conference, Blair said the attacks should not lessen the two countries' commitment in Iraq.
"Our response is not to flinch or give way or concede one inch," he said. "We stand absolutely firm until this job is done," he said.
"Our mission in Iraq is noble and it is necessary," said Mr. Bush. "No act of thugs or killers will change our resolve or alter our fate. A free Iraq will be free of them."
The president even suggested that U.S. troop levels in Iraq could rise, despite the fact that the Pentagon has announced plans to reduce the American military presence in the country next year.
"We could have less troops in Iraq, we could have the same number of troops in Iraq, we could have more troops in Iraq," Mr. Bush said. "Whatever is necessary to secure Iraq."
Protesters said they were uncomfortable with Blair's close relationship with the U.S. president. Some placards referred to the prime minister as Mr. Bush's "poodle." One read: "Stop the organ grinder and his monkey."
"We're angry that Bush appears to be leading our country," said Ted Edwards, a gas supply worker who took the day off to attend the march. "Why Blair is allying himself to Bush I do not know. It's important to show our anger at Bush and to tell Blair he doesn't represent most people in Britain."
"It is ridiculous," said Betty Gallaccio, 13, who was attending the march with her father. "Bush is coming over here and placing us all in danger from terrorists and we have to pay for it."
The protests, which have been brewing for weeks, did not appear to faze the president, who has said repeatedly that he appreciates the freedom of expression that permits such demonstrations.
"Freedom is beautiful," Mr. Bush, adding he was happy to be in a country where people were allowed to speak their minds freely. "All I know is that people in Baghdad weren't allowed to do this until recent history."