The U.S. Senate went into a rare closed session today after Democrats invoked a seldom used rule to back their demands for greater oversight by the Republican-controlled body, particularly on the Bush administration's use of intelligence in taking the country to war in Iraq.

The unexpected shutdown immediately provoked a furious reaction from Senate Republicans, who denounced it as a stunt and an affront.

The closed session lasted a little more than two hours before Republicans mustered a vote to resume meeting in open session.

The Senate majority leader, Bill Frist (R-Tenn.), said in a floor speech after the open session resumed that he and the Democrats had agreed to form a bipartisan committee of six senators to report on progress by the Senate Intelligence Committee in reviewing prewar intelligence.

Under the order to go into closed session, the galleries were emptied of spectators and journalists, staffers without security clearances were ordered out and senators were required to remove all electronic gear such as cell phones and digital communications devices.

The Senate's Democratic leader, Harry M. Reid of Nevada, initiated the closed session by invoking Rule 21, which was seconded by Sen. Richard J. Durbin (D-Ill.), the minority whip. In a floor speech, Reid declared that "a cloud hangs over this Republican-controlled Congress for its unwillingness to hold the administration accountable" on a variety of issues.

He was particularly incensed about what he said was the refusal of the Senate Intelligence Committee under Sen. Pat Roberts (R-Kan.) to follow up on an investigation of the intelligence that led to the war in Iraq. A report was issued in July last year, but a "phase two" inquiry into how the Bush administration used that intelligence has not been held. Reid accused Roberts of breaking a promise to conduct that investigation in an effort to "provide political cover for this administration," which he said had "consistently and repeatedly manipulated the facts" in making its case to invade Iraq in 2003.

"I demand on behalf of the American people that we understand why these investigations are not being conducted," Reid said. He then demanded the closed session.

Frist angrily denounced the move, charging that "the United States Senate has been hijacked by the Democratic leadership." He told reporters that he has never as majority leader "been slapped in the face with such an affront to the leadership of this grand institution."

Frist called the closed session "a pure stunt" by Reid, Durbin and the Democratic leadership.

"This is an affront to me personally," he said. "It's an affront to our leadership. It's an affront to the United States of America. And it is wrong."

Frist sharply criticized Reid personally, saying he could never trust the Democratic leader again.

Roberts said after the open session resumed, "We have agreed to do what we already agreed to do, and that is to complete as best we can phase two of the Intelligence Committee's review of prewar intelligence in reference to Iraq."

Republicans complained that going into closed session interrupted important business.

But Democrats said they were prepared to invoke Rule 21 daily to put pressure on the GOP leadership for the intelligence investigation and other oversight matters.

Durbin said it was the first time that Rule 21 had been invoked in more than 25 years. Although closed sessions have been held from time to time more recently -- the Senate shut its doors last year to discuss intelligence-gathering -- the previous closures were done by agreement of both parties.

"The purpose of this closed session is to discuss the need for a phase two investigation by the Senate Intelligence Committee," Durbin told reporters after the move. He said this investigation was promised as early as Feb. 12, 2004, yet "nothing has been done."

"It is clear now that the American people were not informed properly before the invasion of Iraq," he said. "Intelligence information was distorted, was misused, and we have seen as late as last week the lengths which this administration has gone to try to silence and discredit their critics of the misuse of this intelligence information."

Durbin added, "We're serving notice on [Senate Republicans] at this moment: Be prepared for this motion every day until you face the reality. The Senate Intelligence Committee has a responsibility to hold this administration accountable for the misuse of intelligence information. They have promised this investigation. We will continue to make this request until they do it."

Under Rule 21, the chamber can be ordered into closed session when any member calls for it and the motion is seconded. No vote is required. Going back into regular session requires a majority vote.

Sen. Charles E. Schumer (D-N.Y.) called the move "historic" and said it was prompted by mounting frustration.

"An administration that can't admit it makes mistakes is bound to make many, many, many more of them," he said. The closed session was needed to "get the majority to implement its proper oversight role," he said. "That relates to the war, and it relates to many, many others issues as well."