Al Qaeda's second-in-command Ayman Zawahiri Thursday threatened more terrorist attacks in the United States and London unless U.S. troops withdraw from all Muslim countries.

"What you have seen in New York, Washington and Afghanistan are only the initial losses," Zawahiri said in a videotape broadcast by al-Jazeera, the Arabic language satellite channel. "If you [the United States] continue the same hostile policies, you will see something that will make you forget the horrors of Vietnam," Zawahiri threatened in the videotape, a Kalashnikov propped against his right shoulder.

Repeatedly pointing a finger for emphasis, the bearded, turbaned Zawahiri, an Egyptian-born doctor and Osama bin Laden's top deputy, said that British Prime Minister Tony Blair shouldered the blame for the July 7 attacks in London that killed 56 people, including four suicide bombers. The tape was released exactly four weeks after the deadly attacks on London's transportation system.

President Bush responded quickly to the Zawahiri tape from his ranch in Crawford, Tex. Bush said that Zawahiri's comments "make it clear that Iraq is a part of this war on terror and we're at war."

Bush said that people like Zawahiri "have an ideology that is dark, dim and backwards. They're terrorists and they're killers. They will kill innocent people trying to get us to withdraw from the world so they can impose their dark vision on the world."

Bush said the United States "will stay the course" and "complete the job" in Iraq.

Meanwhile in London, 6,000 police officers, many of them armed, patrolled the streets of the British capital and London's extensive subway system in an operation designed to reassure jittery Londoners on the four-week anniversary of the attacks. It also came two weeks after failed attacks in London on July 21.

The Piccadilly subway line, the most damaged in the July 7 attacks, was fully operational Thursday for the first time since the bombings.

In a chilling revelation Wednesday afternoon, New York Police Commissioner Raymond Kelly told a group of New York business leaders that the London suicide bombers made their explosives out of everyday items such as hair bleach and detonated them with simple cell phones.

Kelly said the bombers could have gone to a local "hardware store or some beauty supply store for ingredients" for their bombs since the items were readily available in such stores.

"Initially it was thought that perhaps the materials were high-end military explosives that were smuggled, but it turns out not to be the case," Kelly said in a wide-ranging briefing to city business leaders.

NYPD officials said investigators in London believe the bombers used a peroxide-based explosive called HMTD, or hexamethylene triperoxide diamine. HMTD can be fashioned out of ordinary ingredients including hydrogen peroxide or hair bleach and types of citric acid.

HMTD breaks down at room temperature, so investigators said it was preserved in big refrigerators, according to Michael Sheehan, deputy commissioner of counterterrorism at the NYPD.

"In the flophouse where this was built in Leeds, they had commercial grade refrigerators to keep the materials cool," Sheehan said.

The NYPD officials, who had been briefed by their counterparts in London, also said British investigators believe the three bombs that exploded in the London subway [one exploded on a bus] were detonated by cell phones that had alarms set to 8:50 a.m. and that the bombers transported the explosives in coolers in the back of two cars.

They said that similar "explosive compounds" were used in the attempted attacks two weeks later but that those bombs were hand-activated rather than timed.

Sheehan said New York police believed that similar attacks could happen in New York City.

The Associated Press contributed to this story.