Nearly two years after the worst terrorist attack on the United States, the Justice Department is on the defensive over charges that a key piece of legislation passed in response to the September 11 attacks threatens civil liberties by giving law enforcement too much power.

The USA Patriot Act as it is formally known, became law in October, 2001 and gave law enforcement a range of expanded tools designed to protect the nation against future terrorist attacks - powers which previously often had to be approved by a judge on a case by case basis. The measure passed Congress by overwhelming margins but in the time since has drawn sharp criticism and lawsuits from a range of groups including civil liberties organizations which say it grants too much power to federal agents.

"Many members of Congress who voted for it didn't even have a chance to read it," said David Keene, the chairman of the lobby group called the American Conservative Union.

The act lifted restrictions placed on the FBI's ability to wiretap suspected terrorist suspects, in many cases removing the requirement that federal agents first seek a court order to conduct electronic eavesdropping.

"What we are doing is creating a situation in which traditional constitutional safeguards are being lowered in the name of fighting terrorism and then by lowering them, we're giving law enforcement power to go after all kinds of other people with tools they previously didn't have," said Mr. Keene.

But law enforcement officials now consider the Patriot Act a cornerstone in the nation's ability to battle terrorism. In the face of new legal challenges and efforts in Congress to revise the law, Attorney General John Ashcroft is set to begin a series of speeches around the country, most if not all of them to law enforcement groups, to defend the act. He began with an appearance Tuesday before a conservative advocacy group in Washington.

"It is critical for everyone to understand what the Patriot Act means for our success in the war against terrorism," said Mr. Ashcroft. "We have used these tools to prevent terrorists from unleashing more death and destruction on our soil. We have used these tools to save innocent American lives. We have used these tools to provide the security that ensures liberty."

But some key members of Congress are now expressing concern that these tools may lead to an erosion of civil liberties. In fact, legislation introduced by Republican Congressman Butch Otter would roll back some provisions of the Patriot Act, including those pertaining to how searches of private property can be conducted.

"The Otter amendment is one that would roll back what's called the sneak and peak provision which allows the FBI to search a person's home or office without telling them until much later," said Jameel Jaffer, a spokesman for the American Civil Liberties Union. "Another that we have real concerns about is section 215 of the act which authorizes the FBI to demand any organization turn its records over to the government and that's a provision that the attorney general himself has said could be used even to get genetic information."

Still, Attorney General Ashcroft considers the Patriot Act an indispensable tool of law enforcement, especially at a time when his top priority has become preventing another terrorist attack on the nation.

"We have neutralized alleged terrorist cells in Buffalo, Detroit, Seattle and Portland," he emphasized. "To date, we have brought 255 criminal charges, 132 individuals have been convicted or have pled guilty. All told, more than 3,000 suspected terrorists have been arrested in many countries. Many more have met a different fate."

The debate over balancing counter terrorism while protecting civil liberties is unfolding at a time when the United States is about to mark the second anniversary of the September 11 attacks, and amid frequent warnings that the war on terrorism as evidenced by Tuesday's suicide bombing at the United Nations mission in Baghdad is far from over.