The Liberal Democrats are set to "rattle the cage of British politics" when Gordon Brown becomes PM.
Sir Menzies says he will put Gordon Brown "in the dock" as PM
Leader Sir Menzies Campbell said his party would "scrutinise his policies with relentless, forensic vigour".
He dismissed the chance of Lib Dems being in a Labour Cabinet, saying there was a "chasm" between parties.
And Sir Menzies, 66, said he would "absolutely" make his age an issue in the battle with Mr Brown and Tory leader David Cameron.
"Politics would benefit from more people with experience," he said in a speech to party staff.
"If more experienced politicians had taken the decisions, we might not be mired in the conflict that we see in Iraq today."
Speaking one day before the chancellor takes over from Tony Blair, Sir Menzies promised to "put him in the dock and scrutinise his policies and pronouncements with relentless, forensic vigour. His fingerprints are all over the scene of the crime".
He said the party would "expose the government" on issues such as freedom of information, civil liberties, and the BAE sale of arms to Saudi Arabia.
"Under a Brown premiership, the Liberal Democrats will rattle the cage of British politics.
"We will give a political voice to those who are not being heard in government."
Addressing the possibility that Liberal Democrats could be included in the next Cabinet, Sir Menzies said the party was not "prepared to trade our principled opposition for the sake of ministerial jobs".
Northern Ireland post
It emerged last week that Mr Brown had offered ex-Lib Dem leader Lord Ashdown the post of Northern Ireland secretary.
Sir Menzies said there was a "political chasm" between the parties on issues such as nuclear energy, ID cards, city academies, council tax, nuclear weapons and the war in Iraq.
Last week, Lord Ashdown said he had been offered the Northern Ireland secretary job after Sir Menzies had said no member of his party would join Mr Brown's government.
But on Sunday Mr Brown said: "This is not a correct account of what happened. It is a total travesty of what happened."
The chancellor, who replaces Tony Blair as prime minister on Wednesday, also told the BBC: "What I would say to you is absolutely this: that there are people of expertise and wide experience.
"They may not be in your own political party, but in the case of Northern Ireland I was talking to someone who had something to offer, and I think it's right that we are inclusive and it's right to draw on all the talents."