Labour is "staking out the new centre ground in British politics", Jeremy Corbyn has said as the party leaders gave their new year messages.
Mr Corbyn said the prospect of a "new Britain" was "closer than ever" and he was leading a "government in waiting".
Prime Minister Theresa May used her new year message to say Britons will feel "renewed confidence and pride" in 2018.
Lib Dem leader Sir Vince Cable called for a second public vote on Brexit and "a real cash injection for the NHS".
Labour defied predictions of a landslide defeat in the 2017 general election to secure a hung parliament.
The result saw the prime minister lose her Commons majority, but the Conservatives remain the largest party.
Mrs May acknowledged that 2017, during which she called a snap election, was a challenging year.
"That is true for each of us personally," she added. The prime minister also lost three senior cabinet members to scandal during 2017.
However, she said: "The real test is not whether challenges come; it's how you face them.
"Whether you allow a task to overcome you, or tackle it head on with purpose and resolve."
'Weak and divided'
The Labour leader used his message to attack a "failed system" of governance and "stagnant economy" of wealth disparity run by a "self-serving elite".
The 2017 general election showed "the establishment's secret is out: they're not as strong as they appear" and have "no idea how to fix their broken system," Mr Corbyn said.
"The old political consensus is finished," he added.
He insisted Labour now occupied the new centre ground in British politics.
"We are staking out the new centre ground in British politics, backing the things which most people want but are blocked by vested interests.
"We are a government in waiting, while the Conservatives are weak and divided and stuck in an outdated rut with no new ideas," he said.
Mr Corbyn said Labour's mission was to "give people support and security and use their talents, unleash their creativity and fulfil their hopes".
"The hope of a new Britain, run in the interests of the many, not the few, is closer than ever before.
"Together we can, and we will, deliver it."
'Closer to fairer society'
The prime minister said that while Brexit will be "crucial" in the coming year, it is "not the limit" of the government's ambitions.
It will focus on schools, the police and NHS to change people's daily lives.
Whilst 2017 was a turbulent year for Mrs May, she described it as one of progress, with the first stage of Brexit talks completed in December.
"Most people just want the government to get on and deliver a good Brexit, and that's exactly what we are doing", she said.
Mrs May said 2018 would also see her move closer to her goal of a "stronger and fairer" society.
She said the government will "build more good schools" and more homes, "so housing becomes more affordable", while also working to "enhance our natural environment for the next generation".
'A lot of work to do'
The Liberal Democrat leader, like Labour, used his message to confront "the establishment", saying his party was "unafraid to swim against the tide" and "stand up for the underdog".
Sir Vince, who replaced Tim Farron as leader in July, said his party had "a lot of work to do" in 2018 to persuade the public to get behind them.
In a video message, he said: "There's still time to offer people the choice of an exit from Brexit; there's a big opportunity to win a real cash injection for the NHS and for schools; and to win the argument for thousands more affordable homes."
"All over the country people are getting behind these campaigns by joining and supporting the Liberal Democrats."
He urged his party to "get back to what we're good at" by mobilising grassroots supporters, adding "let's make 2018 our year".