The senators rejected an effort to open the discussions to the public and voted 57-43 to hold the debate behind closed doors.

Monday's debate was adjourned without a vote after four hours. Senators are expected to continue the debate when they reconvene on Tuesday.

Only after motions concerning dismissal of the trial and the calling of witnesses have been argued and debated will the senators vote on them. Correspondents say that could happen on Wednesday.

The motion to dismiss the trial, brought by Democratic Senator Robert Byrd, is expected to fail, as Republicans have said they will use their 55-45 Senate majority to defeat it.

BBC Washington correspondent Stephen Sackur: "The trial has been described as a bizarre situation"

House prosecutors say dismissing the charges without calling witnesses would be "inconsistent with the constitutional standards and harmful to the institutions of our government".

"I ask you to consider the harm he [Mr Clinton] caused, the indignity he brought to the institution of the presidency," Republican Rep Charles Canady said.

White House lawyer Nicole Seligman argued that the case against President Clinton should not be about punishment.

"Punishment will be found elsewhere, judgement will be found elsewhere, legacies will be written elsewhere. None of that will be dismissed. None of that can ever be dismissed," Ms Seligman said.

Nicole Seligman calls for "a sence of proportionality"

"You have heard the charges in full, heard the defence. Now it's the time to define how the national interest can best be served: by extending this matter indefinitely, or ending it now," she said.

Lead prosecutor Henry Hyde, said the motion to dismiss was a slap at the House of Representatives, which approved the two impeachment articles on largely party-line votes in December.

"I sort of feel that we have fallen short in the respect side because of the fact that we represent the House, the other body, kind of blue-collar people, and we're over here trying to survive with our impeachment articles," Mr Hyde said.

Earlier on Monday, a series of negotiations behind closed doors on how to proceed with the case delayed the start of the hearing.

After the brief appearance of the Senate Republican leader, Trent Lott - who said the senators may be close to an agreement - the Senate went into recess for almost two hours.

Republicans also "summarily rejected" a Democrat proposal for final votes on the articles of impeachment by the end of the week, with no chance of witnesses.

The BBC's Tom Carver in Washington: "The feeling is that they'll achieve more behind closed doors"

BBC Washington Correspondent Philippa Thomas says the Republican senators, while wanting to hear witnesses, are very aware of the fact that the public is tired of the trial and feels there is enough evidence for a simple yes or no vote.

The House managers - the Republican prosecution team - say calling witnesses to give evidence is fundamental in any trial.

Lewinsky interviewed

Robert Byrd: Introduced the dismissal motion

To further their argument, they spent two hours interviewing Monica Lewinsky on Sunday. Afterwards, house manager Bill McCollum said she could be "a very helpful witness" if called to give evidence.

But Ms Lewinsky's lawyer said she had told her interviewers nothing that was not already on record.

Democrats say there is no need to call witnesses as they have already provided evidence to the grand jury and special prosecutor Kenneth Starr.

Potential witnesses

Trent Lott talks to the press

If it is decided to call witnesses, statements will initially be taken in private. Senators will then decide whether to have them appear in the Senate giving live evidence.

The key witnesses for prosecutors, if they get the go-ahead, would be Ms Lewinsky, Betty Currie, who was the president's secretary, and Vernon Jordan, Mr Clinton's close friend.

The prosecution says Ms Currie acted as a go-between during his relationship with Ms Lewinsky. Mr Jordan is said to have helped try to find a job for Ms Lewinsky to ensure her silence.