The Sinn Féin leader, Martin McGuinness, was today warned by the chairman of the inquiry into Bloody Sunday that he faced accusations of having something to hide if he refused to answer questions.

Lord Saville made the comments before temporarily adjourning the hearing to give the Sinn Féin MP an opportunity to consult his lawyers.

He said that Mr McGuinness had refused to say whether he had been a member of the official IRA, when he became commanding officer of the Provisional IRA in Derry, and whether former Belfast councillor Sean Keenan had been explosives officer.

He added that Mr McGuinness had also not been prepared to state in which house he had instructed IRA members not to attack British troops on Bloody Sunday.

Lord Saville said that he failed to understand why Mr McGuinness would not answer these questions when he had told the tribunal he wanted to get to the full truth of what happened.

"I would ask you to consider these questions. If you do not, two things will happen," he said.

"Firstly, you are depriving us of the opportunity of discovering the full facts and matters relating to Bloody Sunday, and secondly, it will be suggested, in due course, that if you are not answering these questions you have something to hide."

After a short adjournment, Lord Saville assured Mr McGuinness's lawyer, Richard Ferguson, that the Sinn Féin leader's immunity to prosecution extended to the question of whether he had been a member of the official IRA and when he became officer commanding the Derry brigade.

Mr McGuinness then told the inquiry that he had joined the official IRA for a few weeks at the end of 1970, before leaving to join the Provisionals.

The Provisionals split from the old official republican movement in 1969, becoming the dominant militant faction.

"At the time of Bloody Sunday I was adjutant in the Derry command of the IRA. Within two weeks of Bloody Sunday, I became officer in command of the Provisional IRA," he added.

However, Mr McGuinness continued to refuse to answer questions about the location of houses in which IRA members had met or stored weapons.

"I have never, ever on any occasion given the name of a single person who was associated with me or the IRA," he said. "To do so would have been a gross act of betrayal.

"I do understand that we are in a different situation. I am not in an interrogation centre, but I have a duty, in my view, stretching back 30 years to these people, and I am not prepared to break my word."

Lord Saville said he understood this to mean that his duty overrode the desire of the families to get to the truth behind Bloody Sunday.

Earlier, Mr McGuinness had criticised the line of questioning, claiming that the inquiry was straying away from its remit of finding out what had happened on Bloody Sunday.

After refusing to reveal when he became the commanding officer of the Derry brigade, he said: "The reason I won't answer it is because it is not relevant to Bloody Sunday, and that question opens up a whole other debate which has significance for the peace process."

Mr McGuinness told the inquiry that he had ordered IRA men not to attack British troops on the day.

He said that his commanding officer had instructed him to tell every IRA volunteer not to open fire during the civil rights march which culminated in 13 unarmed civilians being killed by paratroopers in 1972.

"Without exception, everyone I spoke to accepted that our approach to the march was sensible," he said.

Mr McGuinness also denied accusations that he had fired the first shot.

He further denied accusations by Paddy Ward, a former IRA member, who last month told the inquiry that Mr McGuinness had supplied detonators for 16 nail bombs to be used in a planned attack on troops after the march.

Mr McGuinness told the inquiry, which is sitting in Derry's Guildhall, that Mr Ward's claims were a "tissue of lies".

He added: "Mr Ward is a fantasist, Mr Ward is a liar, Mr Ward is an informer. I did not know Mr Ward. I never met Mr Ward."

Asked whether he had encouraged former IRA members to testify before the tribunal, Mr McGuinness said: "I haven't sought any of them out: some have come to me.

"I made it very clear that I was going to the tribunal, that I was going to give my testimony, and that I was encouraging, both privately and publicly, anyone who can contribute to the uncovering the truth about what happened.

"I told anyone who came to me that they should make their own decision, but I was not in any way going to put pressure on people to take a course of action which, in my view, could only be decided by themselves."

Six other Provisional IRA members have either given statements to the inquiry or promised them, and are expected to give evidence before Christmas.

Mr McGuinness said that when he became aware of the shootings of civilians on Bloody Sunday, his first instinct was that he should get a weapon.

"Whenever it became clear to me that people were being shot in the Glenfada park area, I emotionally felt that I should go and get a rifle and try to defend, as best as I possibly could, the people of the Free Derry area.

"But, of course, I didn't do that. I had a period of reflection, and I think came to a mature decision that it would have been a mistake."

Mr Clarke asked whether the IRA had been engaged in a campaign of destruction of business properties to put pressure on the army to withdraw from the city.

The Sinn Féin leader replied: "It's true that there was a war situation. The primary purpose of the IRA was to take on the British army and those military forces supporting them.

"There was a strategy to attack business premises in order to stretch the army and gain maximum advantage over what were superior - numerically, that is - forces."

The £150m inquiry is drawing to a close, with December 19 scheduled to be the last day of evidence.

Lord Saville and a group of Commonwealth judges began their investigations into Bloody Sunday more than three years ago. He is expected to produce his report by the end of next year.