Four weeks after suicide bombers struck the British capital, thousands of police took to the streets today to reassure jittery Londoners as al-Qaeda warned there would be more attacks.
Underground rail stations swarmed with police, many of them armed, as millions of commuters headed into work.
"It is certainly a very big police operation," said Deputy Chief Constable Andy Trotter. "We are out there to reassure Londoners and also to deter any further attacks."
Al-Qaeda's second in command Ayman al-Zawahri issued a warning to Britain in a new video aired today.
"Blair's policies will bring more destruction to Britons after the London explosions," al-Zawahri, Osama bin Laden's deputy, said in a tape aired by al-Jazeera television.
British Prime Minister, Tony Blair, has consistently rejected the idea that Britain's involvement in the war in Iraq is linked to the attacks on London last month.
Today, the Piccadilly underground railway line was fully operational for the first time since four suicide bombers killed 52 people on three trains and a bus on July 7.
Offering an intriguing glimpse into the investigation, New York police chief Raymond Kelly said the bombs were made from simple ingredients like hair bleach. Three of them were set off by mobile phones, he said.
"It's more like these terrorists went to a hardware store or some beauty supply store," Mr Kelly said.
According to US media reports, he said the explosives were stored in refrigerators and then shipped in coolers to a railway station outside London where the bombers took a train to the capital.
His briefing, partly based on information from New York officers sent to London to monitor the police inquiry, was the first detailed account of the methods used by the bombers.
In another wave of attacks two weeks after the first, four bombs failed to detonate and, after the biggest manhunt in British history, four suspects were arrested, including one seized in Rome.