Two human rights groups have criticised plans to increase the time terror suspects can be held without charge.
Jack Straw said the government was not acting for "macho reasons"
In separate reports, Amnesty International UK and Justice say there is no justification for extending the current 28-day time limit.
Ministers say increasingly complex plots mean a change is needed and are thought to be drawing up safeguards to allow detention of up to 56 days.
Amnesty said any extension would harm relations with the Muslim community.
In a list of 10 points, the Amnesty report also says any extension would undermine basic human rights and devastate the lives of those detained.
Kate Allen, the group's director, said that locking up suspects for two months without charge was "no way for a liberal democracy to act".
"Extremists want to undermine our liberties and terrorism shows no respect for human rights," she said.
"The government needs to step up and show a principled opposition to terrorism, not take away peoples' rights."
Justice argues that in 10 high-profile terrorism cases in the US over the past five years, charges were laid within 48 hours of a suspect being detained.
These included plots to destroy Sears Tower and Brooklyn Bridge, it adds.
Eric Metcalfe, Justice's director of human rights policy and the author of its report, said: "If the FBI can charge a terror suspect in 48 hours, why do UK police need more than 28 days?"
Shadow home secretary David Davis said the Justice report "drives a coach and horses through government claims that we need an extension of the period of detention".
But Justice Secretary Jack Straw said any extension of the 28-day limit would be "very proportionate".
He told BBC One's Andrew Marr show: "We are not doing this for macho reasons, we are doing this because of the scale of the threat as we anticipate it."