"My first reaction was complete fear and horror at the thought of lives being lost," she says. "And then subsequently a fear for what some of the responses might be, and a sense of pressure and responsibility as the incumbent of this job."
The director of Liberty, who began work at the civil liberties organisation the day before 11 September 2001, says she experienced a similar "gut-wrenching feeling" on Thursday as after the attack on New York and Washington.
She is adamant that London's police and security services must now be given the resources - diverted from the ID cards scheme - to catch "the murderers". But she also cautions against a "knee-jerk" political response which could see people stripped of basic rights and freedoms in the name of fighting terror.
Ms Chakrabarti, a former Home Office lawyer who worked with ministers on counter-terrorism policy, warns against a disproportionate reaction from politicians desperate to "be doing something".
"These are emotional moments, and there is a fear that we could creep further into a more authoritarian climate than we have already," she says. So far, Ms Chakrabarti says she has been heartened by the "statesmanlike" reaction of Charles Clarke, the Home Secretary, who has resisted pressure for a crackdown or for activating indiscriminate stop-and-search powers. He has even said publicly that ID cards would not have prevented the atrocity.
"This is the worst possible time for any home secretary and in my view he has really passed the test," she says. She extols Mr Clarke for displaying "underlying instincts of balance and restraint and respecting rights and freedoms, and public protection."
Ms Chakrabarti, 36, a human rights barrister, has not always seen eye to eye with home secretaries on terrorism. Since she took over as director of Liberty, she has attacked the Home Office over the internment without trial of terrorist suspects in Belmarsh prison and over provisions, such as control orders, in the Prevention of Terrorism Act.
She agrees counter-terror policy must mitigate against risk, but that should include "risk to our way of life". "That means keeping in place the fundamental freedoms that make Britain different from the authoritarian societies the terrorists want to create."
Ms Chakrabarti has lived in London all her life. The daughter of immigrants from Calcutta, she contends that the terrorists will have scored a victory if the Government's reaction to the atrocities is stripping freedoms from ordinary people, or arbitrarily targeting people from Muslim communities. "If what the terrorists want is a war of the worlds, the way to give them their desire is to round up people of a particular faith or minority and intern them without trial. And the way to give them that victory is to shut down free speech and every semblance of an open society."
The worst outcome would be if the Government, perhaps unconsciously, does "what the terrorists failed to achieve but letting it occur by indirect means".
"This isn't just rhetoric. That is at the heart of this. Because I am assuming that the people who perpetrated this did it because they want our society to descend into chaos and ultimately tyranny - because our democracy to them is somehow unacceptable."
Ms Chakrabarti cuts a slight figure at just over five feet but she exudes a fearlessness that has made her the country's most listened-to advocate of civil liberties. Speaking from her windowless basement office, she argues that human rights do not need to be sacrificed to catch the bombers. In fact, protection of civil liberties could be integral to finding the attackers.
If Muslim communities are targeted, she argues, and young Muslim men rounded up, this will serve to alienate the people who could aid the authorities in finding the terrorists.
"Visible injustice fuels terrorism or, if not direct terrorism, those who look the other way," she remarks.
She says community relations are "part of the solution" to finding the killers. "You need active societal and community involvement. The way to achieve that is not by more laws that will target particular groups but by inspiring people with the confidence that they are not the enemy, they are not a suspect community."
The director of Liberty says the disproportionate use of existing stop-and-search powers has already engendered a distrust of the authorities among some law-abiding Muslims. "I have met people since I became director of Liberty who hate terrorism and value human life but feel distinctively uncomfortable reporting suspicions, even of terrorism, to the police and the security services."
More worryingly, the heavy-handed use of anti-terrorist powers could act as a recruiting agent for terrorist groups. Questioning must be based on reasonable suspicion, not woolly supposition, she says.
Ms Chakrabarti fears that Tony Blair may try to capitalise on a climate of fear to push through the ID cards Bill. But the bombings, far from diluting her opposition to the Bill have "hardened" her belief that ID cards are wrong.
The attacks have convinced her the billions earmarked for ID cards should immediately be channelled into counter-terrorism, including more police. "If you have a suicide bomber who is prepared to wreak this kind of havoc I don't see how the identity trail is going to be a great deterrent."
"I am not going to say that an ID card will never be useful to anyone in law enforcement ever. But terrorism is the area of law enforcement where I find it particularly difficult to see the benefits of the ID card."
She highlights fears that, as in France, ethnic minorities will be targeted by the authorities and questions if the cards would serve any practical service in tackling terrorism.
She adds that ID cards "represent a fundamental change in the relationship between the individual and the state."
Casting a glance at the portrait of Liberty on her office wall, Ms Chakra- barti pulls her swivel chair closer and declares earnestly: "If I really believed that ID cards could prevent what has just happened from happening again, I might sign up to them. But I just don't see it. The money and the energy could be better spent - and lots of security experts agree with me."Reuse content