Senior legal figures are worried that "inalienable rights" could swiftly disappear unless Tony Blair ceases attacking the judiciary and freedoms enshrined in the Human Rights Act.

Lord Ackner, a former law lord, said there was a contradiction between the Government's efforts to separate Parliament and the judiciary through the creation of a supreme court, and its instinct for directing judges how to behave. He cautioned against "meddling" by politicians in the way the courts operate.

"I think it is terribly important there should not be this apparent battle between the executive and the judiciary. The judiciary has been put there by Parliament in order to ensure that the executive acts lawfully. If we take that away from the judiciary we are really apeing what happened in Nazi Germany," he said.

Lord Ackner added that the Government's proposals to hold terrorist suspects for three months without charge were overblown. "The police have made a case for extending the two weeks but to extend it to three months is excessive."

Lord Lester QC, a leading human rights lawyer, expressed concern that the Government was flouting human rights law and meddling with the courts.

"If the Prime Minister and other members of the Government continue to threaten to undermine the Human Rights Act and interfere with judicial independence we shall have to secure our basic human rights and freedoms with a written constitution," he said.

Lord Carlile, a deputy High Court judge, warned against the whittling away of historic civil liberties. "We have to be acute about protecting what is taken for granted as inalienable rights. In the United States the Patriot Act included a system whereby a witness to a terrorist incident can be detained for up to a year. This is in the land of the free."

The senior barrister remarked that judges had now replaced MPs as the defenders of basic human rights.

"People use d to look to their MPs as the first port of call to deal with any perceived injustice by the executive. Now there is an increasing tendency for people to look to the judges to protect their liberties," he said.

Mark Oaten, the Liberal Democrat home affairs spokesman, said Tony Blair was transforming Britain into an authoritarian state. "In eight years he has dismantled centuries of judicial protection. Britain's reputation as the world's most tolerant nation is now under threat," he said.

If Mr Blair's proposed terror legislation was unamended, said Anthony Scrivener QC, "Britain would be a significant step closer to a police state". The Prime Minister spoke of "summary justice", said the lawyer: "It would be better named street justice."

This week the Law Lords will consider whether evidence obtained under torture abroad should be admissible in British courts. Shami Chakrabarti, director of Liberty, said admitting such evidence would undermine one of Britain's basic freedoms.

"The Prime Minister is trying in his own words to try to tear up the rules of the game," she said. "The rules of liberal democracy are about no torture, free speech and fair trials. Every time he denigrates these he undermines the fabric of our society."