But the White House legal team waived its right to interview her, instead offering a statement of regret from the president for what she has gone through in the past year.

White House officials said lawyers asked no questions because nothing she said required elaboration.

Her videotaped evidence, given privately in a Washington hotel, will be replayed to senators on Tuesday.

Senators who monitored the six-hour testimony of the former White House intern were barred by Senate rules from discussing anything they observed.

The BBC's Tom Carver: "Obstruction of justice is now the key focus"

House of Representatives prosecutors had said they would not focus on her affair with President Clinton, but rather on allegations that efforts were made to obstruct justice after the affair.

Her testimony came as lawyers for the president announced they were taking legal action against Kenneth Starr - the independent counsel who led the impeachment investigations.

They are filing a court complaint accusing Mr Starr of leaking material about the impeachment case to the press. White House lawyer David Kendall described the leaking as illegal and partisan.

Mr Starr promised an investigation into the allegations that someone associated with his office leaked secret grand jury information to the New York Times.

In a statement, the Independent Counsel said, "We are deeply troubled by yesterday's New York Times report. This office has no desire to inject itself into the constitutional process underway in the Senate."

The BBC's Stephen Sackur: "Another constitutional crisis may be looming"

The newspaper had reported that Mr Starr was considering seeking the president's indictment on criminal charges before he leaves the White House in January 2001.

Democrat Senator Joseph Lieberman told ABC's This Week programme that there was no legal precedent to indict a serving president. But he said it was a useful reminder that the president is "subject to the rule of law".

Public may see Lewinsky

Ms Lewinsky is the first of three witnesses to be questioned, as Republicans continue their push to remove Mr Clinton from office.

The BBC's Stephen Sackur: "No Senators believe Bill Clinton runs the risk of losing his job"

Senators are expected to take a vote later this week on whether to make any of her testimony public.

If so, this would be the American people's first chance to see the 25-year-old former White House volunteer talk about her affair with the president.

The session was due to be overseen by six Senators - three Democrats and three Republicans.

Ms Lewinsky has already been questioned 22 times by Mr Starr and was informally interviewed by Republican prosecutors from the House of Representatives last week.

Worried, but likely to survive

The BBC Washington correspondent says it was thought unlikely that any new evidence would emerge from the testimony, but prosecutors were hoping to elicit some detail that would strengthen their case.

Lawyers are also due to question Mr Clinton's friend, Vernon Jordan, and a White House aide, Sidney Blumenthal.

Mr Jordan is expected to testify on Tuesday, with Mr Blumenthal being questioned on Wednesday, in a special soundproof room inside the Capitol - usually set aside for highly sensitive national security matters.

Conviction doubts

The testimony comes amid continuing signs that Republicans will fail to muster the votes necessary to remove Mr Clinton from office.

Democratic Senator Joseph Lieberman: "No legal precedent to indict the president"

Republican Senator Tim Hutchinson said: "The sense right now in just listening to members talk is that there are Republicans who either are not going to vote for perjury or the obstruction of justice charge."

He declined to estimate how many might vote to acquit, but said getting even 50 votes for conviction in the Republican-controlled chamber is in doubt.

During Sunday, some Republican senators appearing on US political talk shows suggested that there may be room for a double-vote scenario - a move mooted last week.

This would involve a "finding of fact" that the president did perjure himself - but then allow senators to vote against removing him from office.