Attempts to introduce "illiberal" measures such as identity cards will meet staunch opposition from the Lib Dems, Charles Kennedy has warned.
Mr Kennedy says all parties are minorities now
After the Queen's Speech, Mr Kennedy said there had to be a "liberal voice".
He laid the path for battles in the House of Lords by saying peers could ditch historic convention and oppose Labour manifesto commitments.
Tony Blair accused the Lib Dems of trying to be different things to different people.
And as Mr Kennedy began a policy review, Conservative leader Michael Howard said Lib Dem promises had not proved "durable".
In the wake of last week's election result, Mr Kennedy argued that all the main parties were now "competing minorities".
"Where we see Labour politicians reconfirmed on such a marginal mandate introducing illiberal measures, they can be assured that they will receive a liberal response from these benches for the good and the quality of our politics as a whole," he said.
Mr Kennedy said ministers should not rely on the Salisbury convention, which says the Lords should not oppose government manifesto pledges.
"It is absolutely ridiculous that this government should now be falling back on a 60-year-old convention relating to absolutely different political circumstances to justify what it's putting forward in its Queen's Speech today," he said.
As well as opposing ID cards, Mr Kennedy said he hoped for an "all-party consensus" when anti-terror laws are reviewed.
"Key decisions of individual liberty should never be in the hands of an over-mighty executive, but should be within judicial control," he said.
The Lib Dem leader also called for "sane debate" on how to restore certainty to pensions in the UK.
The government says it will bring in a draft pensions bill once its commission on the issue reports.
A key plank of the Lib Dem election campaign was the party's plan to replace council tax with a local income tax.
Mr Kennedy admitted some people had not liked the proposal but said the government would have to reform the council tax.
He urged ministers to foster an informed debate and not be quick to reject sensible ideas.
A referendum on the draft European constitution is expected next year and Mr Kennedy predicted this Parliament would be "very much dominated" by the Europe issue.
"It is time that we make a positive case for Europe, one that actually engages and persuades the public," he added.
Mr Blair said the Lib Dem policy review meant the party now had a "policy blank page" which was more "in keeping" with its politics.
"They run to the right of Labour in Tory constituencies, to the left of Labour in Labour constituencies and in this Parliament we are going to make them choose," he said.