Sir Menzies Campbell said he was impatient to see the Lib Dems take national power, as he launched his campaign to lead the party.
Sir Menzies said his party was serious about politics
He said he wanted to stop the poorest people paying tax and tackle Tony Blair's "authoritarian" government.
The 64-year-old acting leader denied being too old for the job.
Asked if he looked "dated" compared to 39-year-old Tory leader David Cameron, he said it was about "open minds not open-necked shirts".
Simon Hughes and Chris Huhne are also standing. Mark Oaten has pulled out.
Launching his campaign at a Westminster restaurant, Sir Menzies said his party was serious about politics.
"Under my leadership, the Liberal Democrats will not be making polite interjections from the sidelines: we will be hammering on the doors of power," he said.
Sir Menzies said Mr Blair would not be in Downing Street for much longer and his mission would be to ensure the Lib Dems became the rallying point and catalyst for a new political era.
"I believe that there should be no glass ceiling for our party, no limit on our aspirations, and no anchor on our ambitions," he added.
Earlier, Sir Menzies told BBC News his priorities were the environment, poverty, personal liberty and internationalism, specifically the decision to go to war with Iraq "in defiance of international rules and the UN".
He said it was up to people to judge for themselves whether this was left or right, saying that the one thing he knew for certain was that the policies were liberal.
His response to questions about Mr Cameron was "bring him on".
Asked about Mr Cameron's claims to be a liberal, Sir Menzies pointed to the last Tory election manifesto and Mr Cameron's voting record, saying people should be "judged not by what they say but by what they do".
The Lib Dem foreign affairs spokesman declined to say whether he stood to the liberal left or the economic liberal side of the party.
He will produce a detailed manifesto next week.
But he said he was not in favour of more grammar schools, but did support a greater degree of "local" influence for the public in choosing schools.
This "localism" was also important in the NHS, where he said the use of private facilities should not detract from a service which should be "free at the point of use".
Nominations close next week for Lib Dem MPs wanting to stand for the leadership of the party.
There are three declared candidates for the contest which culminates in a postal ballot of the party's 73,000 members.
Mark Oaten, who had been planning to stand, pulled out on Thursday.
A new leader will be named on 2 March, a day before the party's spring conference opens in Harrogate.