While Cameron is away Clegg targets Trident

Nick Clegg exacerbated coalition tensions over Trident as he said the "huge, huge" £20bn cost of renewing the nuclear weapons system was hard to justify when benefits were being cut. He spoke out as he delivered a passionate defence of the Government's record on the eve of its 100th day in office, arguing it was proving to be "radical and reforming".

The Deputy Prime Minister also struck an upbeat note in his first appearance since assuming charge of the day-to-day running of the administration while David Cameron takes his summer break.

He cautioned against excessive gloom over the tough economic times ahead and insisted there was "light at the end of the tunnel" for the country.

Speaking at a town hall-style event in central London, his comments on Trident cast a fresh spotlight on one of the most contentious decisions facing the Con-Lib Dem administration.

Tory ministers Liam Fox, the Defence Secretary, and George Osborne, the Chancellor, have already clashed publicly over where the money for renewing Trident would be found.

Mr Clegg reiterated his party's hostility to replacement – and chose emotive examples to argue his case.

"I think the priority within the defence budget should be absolutely to make sure that our brave troops, our brave servicemen and servicewomen, particularly now on the front line in Afghanistan, have what they need," he said.

Mr Clegg, who said he was not altering his views on Trident just because he was in office, suggested it was the "kind of technology and hardware that we acquired as a country in the past, in an era of Cold War conflict". He said Britain's role in the world was changing very fast and had to be "reflected in the kinds of things we spend money on".

Mr Clegg added: "Not to mention the fact that, of course, it's going to be difficult for someone who is going to receive less housing benefit because of the changes we are introducing to understand why, at the same time, we should spend huge, huge amounts of money in a hurry on replacing Trident in full."

The issue of Trident was one of the areas where the Tories and Liberal Democrats agreed to disagree three months ago, with the coalition agreement acknowledging that Mr Clegg's party would "continue to make the case for alternatives".

Despite coalition plans to set an annual cap on the quantity of non-European Union migrants coming to Britain, he told the meeting yesterday that there was no "magic number" for immigration.

He said: "Much more important than that [is] you've got to make sure the immigration system has people's confidence and has people's trust."

The Liberal Democrat leader claimed that the Tory-Lib Dem administration's record to date was proving the sceptics wrong by demonstrating a "strong sense of purpose".

"I think many people felt that a Coalition Government by definition would be some sort of insipid mush, where different parties sort of haggle constantly with each other until they produce some lowest common denominator policies that do not really make a difference," he said

"Actually, what we are finding out after 100 days [is] we are being accused of doing exactly the reverse, which is doing things too quickly, too fast, too radical, too reforming."

Asked if he could have imagined a few months ago that he would be taking the helm of government, he simply replied: "No."

Mr Clegg said it would take the fixed five-year duration of the Government to "sort things out" but added: "I think there is light at the end of the tunnel. There is so much gloomy news around, I think there are some glimmers of better news."

Mr Clegg is embarking on a busy schedule of meetings and speeches during his fortnight "holding the fort" for the Government. Tomorrow he will set out plans to boost social mobility.