Rebel Labour MPs have been urged to stand firm in their opposition to a 42-day pre-charge detention period and not play into terrorists' hands.
Amnesty International appealed to potential opponents of the anti-terror measure on the eve of a showdown Commons vote in which Gordon Brown's authority is on the line.
With Labour MPs under intense pressure to side with the Government, the human rights group called on them to resist succumbing to "the climate of fear that terrorists seek to breed among us".
More than 7,160 people have signed a petition on the Number 10 website, organised by Amnesty, opposing any increase in the current 28-day limit to detention without charge.
In a letter to backbench MPs thought pivotal to the outcome of the vote, Amnesty's UK director, Kate Allen, said the vote represented a watershed for human rights in the UK.
About 50 Labour MPs have expressed concern about the measures, threatening to join the Tories and Liberal Democrats in opposing the 42-day measure in the Counter-Terrorism Bill.
While the Government has tried to buy off rebels with assurances about when it would be invoked and how early MPs would be given a say, Ms Allen insisted the concessions were inadequate.
"All states have an obligation to act to protect people from terrorism," she said.
"The perpetrators of terrorist attacks must be brought to justice. However, unless governments respond to the threat of international terrorism with measures that are fully grounded in respect for human rights, they risk undermining the values they seek to protect and defend.
"You have an opportunity this week to defend the values that underpin civil liberties in this country. I urge you to stand in support of principles that lie at the heart of our society, principles such as justice and liberty. The alternative is to succumb to the climate of fear that terrorists seek to breed among us."
