WASHINGTON (Reuters) - Former Vice President Al Gore called on Monday for an independent counsel to investigate whether President George W. Bush broke the law in authorizing domestic eavesdropping without court approval.

U.S. Attorney General Alberto Gonzales plans to testify in Senate hearings, expected next month, to give the administration's legal justification for the secret domestic eavesdropping operation.

"A special counsel should be immediately appointed by the attorney general to remedy the obvious conflict of interest that prevents him from investigating what many believe are serious violations of law by the president," Gore said in a speech to the American Constitution Society and the Liberty Coalition.

Gore, the Democrat defeated by Bush in the 2000 presidential election, said the eavesdropping operation threatened the foundation of U.S. democracy, and he recalled the FBI's secret surveillance of Martin Luther King, on the U.S. holiday commemorating the civil rights leader.

Gonzales said in an interview on CNN's "Larry King Live" later on Monday, "I don't know why there would be a need for a special counsel at this time.

"From its inception, this (eavesdropping) program has been carefully reviewed by the lawyers at the Department of Justice and other lawyers within the administration, and we firmly believe that the president does have a legal authority to authorize electronic surveillance in order to gather up foreign intelligence ... of the enemy in a time of war," Gonzales said.

Gore's comments also came at the start of a congressional election year in which Democrats are seeking to regain majority control from Republicans.

He accused Bush of breaking the law for not getting court approval for the National Security Agency eavesdropping operation on communications such as phone calls and e-mail coming into and going out of the United States of people suspected of terrorism ties.

"We still have much to learn about the NSA's domestic surveillance. What we do know about this pervasive wiretapping virtually compels the conclusion that the president of the United States has been breaking the law repeatedly and insistently," Gore said.

"A president who breaks the law is a threat to the very structure of our government," he said.

The 1978 Foreign Intelligence Surveillance Act makes it illegal to spy on U.S. citizens in the United States without the approval of a special, secret court.

Bush has steadfastly said his actions were within the law and that he ordered the domestic eavesdropping operation to fight terrorism after the September 11 attacks.

"Al Gore's incessant need to insert himself in the headline of the day is almost as glaring as his lack of understanding of the threats facing America," Tracey Schmitt, spokeswoman for the Republican National Committee, said in a statement.

"While the president works to protect Americans from terrorists, Democrats deliver no solutions of their own, only diatribes laden with inaccuracies and anger," she said.

The New York Times reported on its Web site on Monday that the American Civil Liberties Union and the Center for Constitutional Rights planned to file separate lawsuits on Tuesday against the Bush administration's domestic spying program.

The two leading civil rights groups will ask the court to immediately order an end to the eavesdropping program, the newspaper said.