In the first episode of the new coalition series of The Thick of It, the Liberal Democrat minister in the fictional Department of Social Affairs and Citizenship was portrayed as a frustrated politician whose great ideas are pinched and ruined by his Conservative superior. It would be tempting to draw parallels between this storyline and the position of Jeremy Browne, the Liberal Democrats' newly promoted Home Office minister, who has found himself with the unenviable task of keeping his Tory counterparts in line while the Government's contentious plans for "web snooping" and "secret courts" come under fire.

But in an interview with The Independent he seemed more than prepared for the fight, insisting he would block any measure which erodes people's civil liberties. Theresa May, the Home Secretary, wants to allow police and the intelligence services to monitor phone, email and internet use. And Kenneth Clarke, the former Justice Secretary, proposed that some civil court cases could be heard in private to protect sensitive intelligence material.

With both measures expected to come under attack from activists at the Liberal Democrat conference, which starts next Saturday, Mr Browne said: "People can rest assured we will be very vigilant defenders of civil liberties.

"Our starting point, the reason I became a Liberal Democrat, and the reason many people joined the Liberal Democrats, is because they want people to be free and empowered and are suspicious of the state taking on excessive authority."

He insisted the internet-surveillance plan, which is now being examined by a parliamentary committee, was not a "done deal". He said: "If the die was cast, [the committee's] work would be redundant. We need to look seriously at what their thoughts are."

Mr Browne, an ally of Nick Clegg's, the Deputy Prime Minister, said: "Liberal Democrats recognise, it is widely recognised, that there are some threats to our national security that it is important for us to take seriously, terrorism being an obvious one.

"We want to make sure we have the tools necessary to protect the public, but it's always worth remembering that the state is a servant of the people, not the other way round. What we have to do is ensure people are free from unnecessary or excessive interference in their lives." He said that "overwhelming evidence" would have to be presented before the party backed any measure that eroded that freedom.

Mr Browne was appointed minister for Crime Prevention in last week's reshuffle as Mr Clegg sought to move ministers into key positions in the Home Office and the Departments of Health and Education.

Party sources have said his appointment was designed to give the Liberal Democrats a louder voice in the Home Office, but Mr Browne said: "When it comes to reducing crime, I want the Home Office to be regarded as a successful department within government and [its] ministerial team to be regarded as a functioning, successful, collegiate team. I don't come here regarding myself as the principal opposition figure embedded in the Government."

Mr Browne, who spent about a third of his time on the other side of the world in his previous post as a Foreign Office minister, said: "Nick Clegg has a limited number of chess pieces to cover the board and he can't have them on every single departmental square.

"My reading of it is that he has consciously lined up ministers of state in the departments which are big domestic departments, dealing with the day-to-day concerns of people across the country. We have a minister of state doing crime, a minister of state doing schools, a minister of state doing hospitals. Beyond the economy as a whole, those are the frontline issues that a lot of people are concerned about."

He accepted that the Liberal Democrats had suffered "growing pains" since joining the Coalition, but he said they remained in "reasonably good health" despite their collapse in the polls and weak performance in many elections. Their spell in office had finally killed the accusation that a vote for the party was wasted as it would never attain power, Mr Browne said.

But he spoke of his fears that some activists still had not adjusted to the reality of being in government. "The danger is they see this as a five-year period to be endured before they can get back to the easier comforts of being an opposition party that criticises other people who do things rather than doing things yourself: the politics of the sit-in protest rather than the politics of implementation in office."