Al Qaeda's second-in-command said in a video statement Thursday that the London transit bombings were retribution for British military intervention in Muslim countries and warned of "more destruction" if Britain doesn't withdraw. He also threatened new attacks on Americans.

The statement from Ayman Zawahiri was issued on a day in which London commuters rode trains and buses amid some of the tightest security since World War II, due to fears of a repeat of the attacks of July 7 and 21. No attack occurred. Instead, the four-week anniversary of the July 7 explosions brought the first statement clearly from al Qaeda about the violence.

Prime Minister Tony Blair "has brought you destruction in central London and, God willing, will bring more destruction," Zawahiri said, addressing the "English people" in a statement broadcast on the al-Jazeera satellite television channel.

"Our message to you is crystal clear: Your salvation will only come in your withdrawal from our land, in stopping the robbing of our oil and resources, and in stopping your support for the corrupt and corrupting leaders."

Officials at the prime minister's Downing Street office provided no comment. Blair has previously rejected claims that the transit bombings were a result of Britain's role in the Iraq war, and politicians across the spectrum have stood by him.

Zawahiri, wearing a white tunic and black turban and posed next to a rifle, did not directly assert al Qaeda's responsibility for the attacks. But he said that since Britons "shed rivers of blood in our land, we exploded volcanoes of anger in your land."

His statement accused U.S. leaders of concealing from Americans that "there is no exit from Iraq except in immediate withdrawal." He called the casualties of Sept. 11, 2001, "merely the losses from the initial clashes."

President Bush, speaking in Crawford, Tex., said that the comments "make it clear Iraq is a part of this war on terror, and we're at war. He's saying, you know, leave. . . . People like Zawahiri have a ideology that is dark, dim, backwards. He's threatening. They have come up against a nation that will defend itself."

British police are questioning at least 17 people in connection with the attacks of July 21, including four men suspected of carrying bombs onto three subway trains and a double-decker bus. Those bombs failed to explode, but startled Londoners into thinking that the July 7 explosions, in which four presumed bombers and 52 other people died, might have been the beginning of a coordinated campaign of violence.

Investigators are attempting to determine if the bombers in the two plots were linked or directed by al Qaeda or some other international group.

"I tend to think it's not al Qaeda-linked, but it's al Qaeda-inspired," said Rime Allaf, a Middle East specialist at Chatham House, a private foreign affairs research center in London. "Al Qaeda's not shy about claiming responsibility."

Zawahiri has issued three previous statements this year by audio or video recording. The CIA and the National Counterterrorism Center were studying the latest, and U.S. officials said it appeared to be authentic. It differs from the past three videos in that Zawahiri wears a black turban rather than a white one and appears to be outside in front of a light-colored cloth rather than in a studio-like setting.

Christopher Blanchard, an analyst who has closely followed al Qaeda's public statements at the Congressional Research Service, called the tape an example of "pragmatic messianism" by Zawahiri. "Al Qaeda's fractured leadership apparently continues to believe that it can undermine support for current U.S. policies by directly addressing and threatening the American people," he said.

The first person to be charged in the bombings was ordered by a London judge Thursday to be held in custody until a hearing on Aug. 11. Ismael Abdurahman, 23, is charged with withholding information from police about one of the suspects in the July 21 attacks.

On Thursday, police charged two London sisters with similar offenses.

In New York on Wednesday evening, Police Commissioner Raymond Kelly told a gathering of business leaders that the bombs were made of easily obtained ingredients, the Associated Press reported. "It's . . . like these terrorists went to a hardware store or some beauty supply store," Kelly said.

Kelly said the bombers transported the bombs in beverage coolers to keep them from degrading, and that the bombs were probably detonated by cell phone alarms. Kelly said the information came from New York detectives who went to London to monitor the investigation.

London police were on extremely high alert Thursday, fearing that other bombers might follow an established pattern. The July 21 attacks occurred exactly two weeks after the July 7 bombings, and Thursday was two weeks after the second attack. Police were stationed at most subway entrances around the city, and public address systems on subway cars repeatedly warned passengers to be vigilant and report suspicious packages or people.

Thursday also marked the first time London's subway was operating at full capacity since the first bombs exploded during morning rush hour on July 7.

In interviews, many subway riders said they were not afraid. "You're more likely to get run over by a bus than to be killed in a terror attack," said Rob Parker, 36, a management consultant catching a train at the St. James's Park subway station. He said the al Qaeda tape was worrisome, but he would not alter his commuting routine because of it.

He did not hold the British government responsible for provoking the violence. "At the end of the day, we're a democracy and we voted to put people in office and make decisions," Parker said. "No one has a crystal ball. I thought that the Iraq war was the right thing to do."

London pollster and political analyst Stephen Shakespeare said the al Qaeda warning would probably boost Blair's political standing, which has already been helped by the government's firm, organized response to the bombings and a wave of national solidarity in the face of attack. The message will undermine critics who say he has overstated the threat, Shakespeare said.

The potential effect on Britain's 1.6 million Muslims is less clear. Shakespeare said his organization's recent polls show that while most feel loyal to Britain, about a third of Muslims polled feel a "divided loyalty" and 5 or 6 percent support the bombings.

Inayat Bunglawala, spokesman for the Muslim Council of Britain, a coalition of moderate Muslim groups, said the tape was probably intended to "incite others to go from being disaffected to taking up bombs."

"But I don't think it will contribute to that," he said. "What will contribute is the policies our country has been engaged in. We have always viewed Iraq as a disaster area for our country. . . . The solution is to remove the sources of anger among Muslim youth."

But Allaf, the Middle East specialist, said she believed the tape would add to the radicalization of Muslim youth in Britain. The most notable aspect of Zawahiri's address, she said, was that it was "logical and lucid" and would appeal to a wider audience than the rhetoric of "crazy mullahs."

Staff writers Susan B. Glasser in Washington and Dafna Linzer in New York contributed to this report.

An undated tape shows Ayman Zawahiri, al Qaeda's deputy leader, at an undisclosed location.An armed police officer patrols Charing Cross station in central London. Four weeks after bombers struck, thousands of police took to the streets in a highly visible security operation.