Sir Vince Cable, the former business secretary, has launched a UK-wide tour after succeeding Tim Farron and becoming the new leader of the Liberal Democrats on 20 July.
Cable, 74, was backed by all 11 other of the party's MPs and had no competition for the top job. He praised Farron, who announced his resignation following the 2017 general election, and welcomed Jo Swinson as the party's new deputy leader.
"There is a huge gap in the centre of British politics and I intend to fill it," he said. "As the only party committed to staying in the [EU's] single market and customs union, the Liberal Democrats are alone in fighting to protect our economy.
"It will soon become clear that the government can't deliver the painless Brexit it promised. So, we need to prepare for an exit from Brexit.
"Theresa May wants to take Britain back to the 1950s while Jeremy Corbyn wants to take Britain back to the 1970s. I will offer an optimistic, alternative agenda to power the country into the 2020s and beyond."
He added: "Under my leadership the Liberal Democrats will be at the centre of political life: a credible, effective party of national government. We have doubled our membership and our new members have given the party enormous energy. I want to give leadership to that energy, hitting the headlines and putting our party at the centre of the national debate."
Cable's appointment comes just over a month after he won his west London seat of Twickenham back, which he had lost at the 2015 general election to Conservative Tania Mathias. Cable will be almost 80 the new parliament runs its full course and the next general election is held in 2022.
"Age should not be a bar," he told IBTimes UK in June. "We got over the stereotyping of women, we got over the stereotyping of black and asian people and the stereotyping of gays. We should get over stereotyping of older people."
But will he be leading the Liberal Democrats into 2022? "I don't have a plan to deal with an event that is almost light years away in terms of politics, an enormous amount could happen in the next few years," Cable said.
"But obviously I've thought about it. After two or three years, once we've got through that critical period, there is a choice: I could stay on or I could hand over to other, younger people. It depends how I feel and they feel at the time." The latest opinion poll from Ipsos MORI, of more than 1,000 people between 14 and 18 July, put the Liberal Democrats on 9%, behind Labour on 42% and the Conservatives on 41%.