(Recasts, adds more quotes and background)
By Doina Chiacu and Patricia Zengerle
March 11 (Reuters) - A dispute between the Central Intelligence Agency and a U.S. Senate committee that oversees it burst into the open on Tuesday when a top senator accused the agency of spying on Congress and possibly breaking the law.
Senator Dianne Feinstein delivered a scathing critique of the CIA's handling of her panel's investigation into a Bush-era interrogation and detention program that began after the Sept. 11, 2001, attacks but was only made public in 2006.
"I have grave concerns that the CIA's search may well have violated the separation of powers principles embodied in the Constitution," Feinstein said in a highly critical speech on the Senate floor by a traditionally strong ally of U.S. intelligence agencies.
She said the CIA searched committee computers to find out how staff obtained an internal agency review that was more critical of the interrogation program than the official CIA report.
"Besides the constitutional implications, the CIA's search may also have violated the Fourth Amendment, the Computer Fraud and Abuse Act, as well as Executive Order 12333, which prohibits the CIA from conducting domestic searches or surveillance," Feinstein said.
CIA head John Brennan denied the allegations.
"Nothing could be further from the truth," he said in a speech at the Council on Foreign Relations think tank.
Feinstein's comments were the latest salvo in a long-running and bitter dispute between the intelligence committee and CIA over the agency's detention and interrogation of terrorism suspects, a program that was phased out when inmates were transferred to Guantanamo Bay Naval Base in Cuba.
The Senate Intelligence Committee's own 6,300-page report criticized some of the harsh interrogation measures used by the CIA, and Feinstein has been pushing to make its findings public.
Feinstein said the internal CIA review mirrored some of the same concerns outlined in her staff's report, unlike the official CIA assessment of the program.
However, as the panel moved close to declassifying some of the information - a move she said was backed by the White House - the CIA acting general counsel went to the Justice Department to complain about committee staff.
"I view the acting general counsel's referral as a potential effort to intimidate this staff - and I am not taking it lightly," she said.
The California Democrat bristled at suggestions her staff had obtained information improperly, and said the CIA itself provided her committee with more than 6.2 million documents.
"The committee clearly did not hack into CIA computers to obtain these documents, as has been suggested in the press," Feinstein said.
After the speech, Senator Patrick Leahy, the senior member of the Senate, said he had never heard a more important speech in the chamber. (Editing by Bernadette Baum)