President Bush's top counterterrorism adviser warned seven days before the Sept. 11, 2001, terrorism attacks that hundreds of people could die in a strike by the al Qaeda network and that the administration was not doing enough to combat the threat, the commission investigating the attacks disclosed yesterday.

Richard A. Clarke, who served as a senior White House counterterrorism official under three successive presidents, wrote to national security adviser Condoleezza Rice on Sept. 4, 2001, urging "policymakers to imagine a day after a terrorist attack, with hundreds of Americans dead at home and abroad, and ask themselves what they could have done earlier," according to a summary of the letter included in a commission staff report. Clarke also cites the same plea in his new book.

Clarke told the commission in testimony yesterday afternoon that whereas the Clinton administration treated terrorism as its highest priority, the Bush administration did not consider it to be an urgent issue before the attacks.

"I believe the Bush administration in the first eight months considered terrorism an important issue but not an urgent issue," Clarke told the 10-member panel. ". . . There was a process underway to address al Qaeda. But although I continued to say it was an urgent problem, I don't think it was ever treated that way."

Clarke's appearance before the panel, formally known as the National Commission on Terrorist Attacks Upon the United States, climaxed days of furor over claims in his book that the Bush administration did not do enough to pursue al Qaeda before Sept. 11, 2001, and has neglected the war on terrorism since then because of an obsession with waging war on Iraq.

The second day of this week's commission hearings also produced new revelations about events before the attacks, including a denial of the White House's long-standing claim that Bush requested a briefing on the domestic threat posed by al Qaeda in August 2001.

But perhaps the day's most dramatic moment came at the start of Clarke's testimony, when he issued an apology that prompted sobs and cheers from the front rows of the packed hearing room, which were filled with relatives of victims of the terrorist attacks.

"To the loved ones of the victims of 9/11, to them who are here in the room, to those who are watching on television, your government failed you," he said. "Those entrusted with protecting you failed you. And I failed you. We tried hard, but that doesn't matter, because we failed. And for that failure, I would ask, once all the facts are out, for your understanding and for your forgiveness."

Administration officials inside and outside the commission's meeting room continued to wage fierce attacks yesterday on Clarke's motives and credibility. The White House authorized identifying Clarke as the official who anonymously gave a background briefing for reporters in 2002 that included positive comments about Bush's anti-terrorism strategies.

Rice, who has refused to testify publicly before the commission, met with reporters late yesterday and said that Clarke has sharply changed his view of the administration's war on terrorism. "This story has so many twists and turns now that I think he needs to get this story straight," Rice said.

She said he never raised concerns with her about the impact of the invasion of Iraq on counterterrorism efforts. Rice also characterized Clarke's Sept. 4 letter predicting deaths from a terrorist strike as a policy document that contained no specific warnings.

The White House released an e-mail from Clarke to Rice sent four days after the attacks that said the White House had warned law enforcement agencies and the Federal Aviation Administration that top counterterrorism officials feared a major al Qaeda attack "was coming and it could be in the US . . . and did ask that special measures be taken."

At the hearings, top officials of the Clinton and Bush administrations also resumed sparring over details of their counterterrorism policies and defending their respective efforts to guard against attacks.

The charged political climate enveloped the commission as well. Key Democrats and Republicans on the panel dropped the neutral posture they had shown in previous hearings and were openly partisan in questioning Clarke and other witnesses. Three GOP members of the group, for example, grilled Clarke on his motivations, suggesting that he had been contradictory in his statements and dishonest in the past about his misgivings about counterterrorism policies.

The drama of Clarke's appearance nearly overshadowed a series of notable disclosures at yesterday's hearing. Among them:

* The CIA now says that a controversial August 2001 briefing summarizing potential attacks on the United States by al Qaeda was not requested by President Bush, as Rice and others had long claimed. The Aug. 6, 2001, document, known as the President's Daily Brief, has been the focus of intense scrutiny because it reported that Osama bin Laden advocated airplane hijackings, that al Qaeda supporters were in the United States and that the group was planning attacks here.

After the highly classified document's existence was first revealed in news reports in May 2002, Rice held a news conference in which she suggested that Bush had requested the briefing because of his keen concern about elevated terrorist threat levels that summer. But Richard Ben-Veniste, a Democratic commission member, disclosed at the hearing yesterday that the CIA informed the panel last week that the author of the briefing does not recall such a request from Bush and that the idea to compile the briefing came from within the CIA.

But a White House official who demanded anonymity replied: "We did request such a document. It's not out of the question that the CIA and others had the same idea."

* Commission investigators disclosed that during the Clinton administration, the president and other White House officials signed a series of secret orders for covert action that, according to the top Clinton aides, authorized the killing of bin Laden by CIA proxies.

But CIA Director George J. Tenet and other agency employees, including those in the field, told commission investigators that they interpreted the orders as requiring them to attempt a "credible capture" of bin Laden and to kill him only if it was necessary as a part of that attempt. When the leader of the Northern Alliance was briefed on the perceived restrictions, he laughed and, according to the staff report, said: "You Americans are crazy. You guys never change."

The report also found that Tenet and others at the CIA never told anyone in the Clinton White House that they felt constrained. Tenet testified yesterday that he would have done so if he had thought it was necessary.

* In the summer of 2001, veteran counterterrorism officers privy to reports on al Qaeda threats "were so worried about an impending disaster that one of them told us that they considered resigning and going public with their concerns," according to one of two staff reports issued by the commission yesterday. Senior CIA officials were also frustrated by some Bush appointees who were not familiar with surges in terrorist threat information and questioned their veracity, the report said.

Tenet said that the death of bin Laden, even in the summer of 2001, probably would not have stopped the attacks on New York and the Pentagon because the plot was already "up and running."

The two staff reports issued yesterday appeared to confirm many of Clarke's key allegations and criticisms, including his assertion that the Bush administration halted use of Predator surveillance drones over Afghanistan to conduct tests on arming the aircraft.

In his testimony, Clarke described the Sept. 4, 2001, National Security Presidential Directive, a strategy for addressing al Qaeda that administration officials have characterized as a bold departure from the Clinton years. But Clarke said the three-stage plan differed little from strategies already in place under Clinton that included first warning the Taliban government in Afghanistan, then pressuring it to turn over bin Laden and finally ousting it through third parties.

It was only after the Sept. 11, 2001, attacks that the introduction of U.S. forces was added, although contingency plans by both the CIA and the Pentagon existed, Clarke said.

Clarke said that he had wanted the directive to say "that our goal should be to eliminate al Qaeda," but that Bush officials called that "overly ambitious." It was reworded to say the goal was to "significantly erode" bin Laden's network. After the Sept. 11 attacks, the word "eliminate" was added back into the directive, he said.

Former deputy attorney general Jamie S. Gorelick, a Democratic commission member, asked Clarke whether Rice's recent statement that the Bush plan "called for military options to attack al Qaeda and Taliban leadership, ground forces and other targets, taking the fight to the enemy where he lived" was accurate.

Clarke responded, "No, it's not."

Staff writer Mike Allen and research editor Margot Williams contributed to this report.

"Those entrusted with protecting you failed you," Richard A. Clarke, right, told Sept. 11 victims' relatives, including Beverly Eckert and Abraham Scott.Victims' relatives applaud Richard A. Clarke's testimony before the panel. "To the loved ones of the victims of 9/11, to them who are here in the room, to those who are watching on television, your government failed you," he said.CIA Director George J. Tenet testified about orders to capture Osama bin Laden.