A former White House security expert has said the Bush government did not consider terrorism to be an urgent threat before 11 September 2001.
The White House has rejected Mr Clarke's allegations
Richard Clarke said that in the first eight months of office, the government considered terrorism "an important issue, but not an urgent issue".
And he said the invasion of Iraq had "undermined" the US-led war on terror.
Mr Clarke was testifying before the US commission examining the 2001 attacks on New York and Washington.
He began his evidence with an apology to the relatives of the roughly 3,000 victims of the 11 September attacks. The US government had failed them, he said.
In a book published this week, Mr Clarke accuses Mr Bush of ignoring warnings before the attacks.
George Tenet, CIA director
Samuel Berger, former national security adviser
Richard Clarke, former counter-terrorism co-ordinator
Richard Armitage, Deputy Secretary of State
In his testimony, Mr Clarke, who quit his post last year, said Mr Bush was told "dozens of times" that there was an urgent terror threat during 2001.
"There was a process under way 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," he said.
Mr Clarke - who has faced a barrage of criticism from the White House over his claims - said Iraq was the reason he was "strident" in criticising Mr Bush.
"By invading Iraq, the president of the United States has greatly undermined the war on terrorism," he told the panel.
Mr Clarke also criticised the FBI for not passing on information that two known al-Qaeda operatives were in the US weeks before the 11 September attacks.
"I had been saying to the FBI... that because of this intelligence that something was about to happen, they should lower their threshold of reporting - that they should tell us anything that looked the slightest bit unusual," he told the panel.
"I would like to think even without benefit of 20/20 hindsight I could have connected those dots."
CIA boss defends record
Mr Clarke served as head of counter-terrorism under four consecutive US presidents, from Ronald Reagan to George W Bush.
He denied claims made by the White House that he was working for Mr Bush's rival in the November presidential election, John Kerry.
"The White House has said that my book is an audition for a high-level position in the Kerry campaign," Mr Clarke said. "So let me say here, as I am under oath, that I will not accept any position in the Kerry administration should there be one."
Earlier, CIA director George Tenet said that, despite intense efforts by his agency to tackle terror threats, the CIA had no prior warning of the 11 September plot.
"We didn't steal the secret that told us what the plot was," he said. "We didn't recruit the right people or... collect the data, notwithstanding enormous efforts to do so."
Asked by the panel why the CIA had failed to prevent the attacks, Mr Tenet said:
"We didn't integrate all the data we had properly, and probably we had a lot of data that we didn't know about that if everybody had known about maybe we would have had a chance."
Later, the US Deputy Secretary of State Richard Armitage defended his government's record.
Asked whether the government had envisaged planes being used for a domestic attack, he told the panel:
"I just don't think we had the imagination required to envisage such an attack."
Also testifying on Wednesday - the second day of public hearings - was President Clinton's former National Security Adviser Sandy Berger.
He said he warned his successor in the Bush team, Condoleezza Rice, that she would spend more time on terror than anything else.
Investigating US policy before, and response after 9/11 attacks
Bipartisan: Five Democrats, five Republicans
Set up by Congress, Nov 2002
Eight public hearings so far, three more to go
President Bush to meet privately with panel leaders
To report findings on 26 July
"I did my best to emphasise the urgency I felt," Mr Berger said.
This is the eighth public hearing held by the bi-partisan commission, established in 2002.
In a preliminary report on its findings so far, the commission said the Clinton and Bush administrations were too slow in moving away from diplomatic pressure to direct military action as a way of dealing with the al-Qaeda leadership.