Charles Kennedy said yesterday he will tell President George Bush that "millions of people" opposed the war on Iraq and are concerned by the "tragic consequences which have followed".

The Liberal Democrat leader, who is due to meet Mr Bush at Buckingham Palace tomorrow, is preparing for a frank exchange of views with the President and will overtly criticise his decision to go to war. Mr Kennedy said yesterday he respected protesters who are planning to march in opposition to President Bush's visit to Britain.

He said he did not want to see a "draconian" crackdown by police but hoped that that the march would be peaceful.

The Liberal Democrats are to send a senior figure, probably Jenny Tonge, the MP for Richmond Park, to speak at Thursday's anti-Bush rally.

Mr Kennedy encouraged people who felt strongly about the military strike in Iraq and its consequences to march to express their views.

"I think we should use the opportunity to leave the President in no doubt as to the extent of public concern, not just in our own country but in Europe generally about the way in which events, tragically, have unfolded," he said on Radio 4's Today programme.

Mr Kennedy said later that he realised that "many people will wish to protest about these issues when the President is here". But he urged protesters to express their views peacefully and said: "There is absolutely no justification for violence in these protests, but peaceful demonstration is a democratic right in our country and must be allowed."

The Lib Dems' leader is planning to raise the issue of British citizens who are being held in Guantanamo Bay by the American military, where they have been blindfolded, chained and kept in "cages".

He has expressed concern in the past about the conditions combatants in Afghanistan are being held in. He has raised the issue with the Prime Minister and has accused Mr Bush of refusing them proper rights as prisoners of war or a fair trial.

Mr Kennedy also voiced concerns about the survival of the coalition in Iraq and the implications of the US seeking an early exit. "There is no way that having made this commitment, Britain or the US can be walking away and leaving a situation of chaos, which could well result in there being a worse situation than the one ... at the outset," he said.