LONDON — Al-Qaida's second-in-command said in a video statement yesterday 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 al-Zawahri 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 bombings of July 7 and 21. No attack occurred. Instead, the four-week anniversary of the July 7 explosions brought the first statement unequivocally from al-Qaida about the violence.

Prime Minister Tony Blair "has brought you destruction in central London and, God willing, will bring more destruction," al-Zawahri 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 offered 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.

Al-Zawahri, wearing a white tunic and black turban and posed next to a rifle, did not directly assert al-Qaida'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, Texas, said 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 al-Zawahri have an ideology that is dark, dim, backwards. He's threatening. They have come up against a nation that will defend itself."

British police are attempting to determine whether the bombers in the two plots were linked or directed by al-Qaida or some other international group.

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

Al-Zawahri has issued three previous statements this year by audio or video recording. The CIA and the National Counterterrorism Center were studying the latest. It differs from the past three videos in that Al-Zawahri 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 setting.

Christopher Blanchard, an analyst who has closely followed al-Qaida's public statements at the Congressional Research Service, called the tape an example of "pragmatic messianism" by Al-Zawahri. "Al-Qaida'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.

In interviews, several London 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-Qaida tape was worrisome, but he would not alter his commute because of it.

He didn't 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-Qaida warning would probably boost Blair's political standing, which has already been helped by the government's firm response to the bombings and a wave of national solidarity in the face of attack.

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 the tape would add to the radicalization of Muslim youths in Britain. The most notable aspect of Al-Zawahri's address, she said, was that it was "logical and lucid" and would appeal to a wider audience than the rhetoric of "crazy mullahs."

Washington Post staff writers

Susan B. Glasser and Dafna Linzer contributed to this article.