Jeremy Corbyn warned that people should write Labour off "at their peril" after making a triumphant return to Peterborough following a victory in last night's by-election.
Mr Corbyn joined his party's winning candidate Lisa Forbes in the city centre to deiiver a message to the country and call once again for a general election.
Speaking from the Guildhall after Ms Forbes had defeated bookies' favourite Brexit Party candidate Mike Greene by 683 votes, he said: "What we did was offer the politics of hope, not the politics of fear.
"We offered the politics of hope to end austerity, to fund our schools properly, to employ our police properly, to give our young people a future in this country. That is what the Labour offer is about.
"Everybody, all the experts, wrote Lisa off. All the experts wrote Labour off. Write Labour off at your peril.
"We are strong, we are very determined, to offer that politics that invests in decent services, in decent housing, in decent healthcare and in good quality jobs for the future, and a relationship with Europe that doen't take us over a cliffedge, that protects jobs, protects investment and gives our young people that future.
"Yesterday, the Labour Party came together on the streets of Peterborough, the Labour Party came together in this campaign and, on the day Theresa May ceases to be leader of the Conservative Party, my message is to all the squabbling contenders of the Tory Party leadership, bring it on. We are ready for a general election at any time.
"And that general election will deliver a Labour government that will work for the many, and Britain will not be a country that in one year alone has handed £50 billion to the tiny majority of very rich people. Instead, it will be a country that cares for all.
"And I'm utterly determined that it's our party that's going to take us to that so much better place."
The by-election had been triggered after Peterborough constituency voters recalled former MP Fiona Onasanya following her jailing for perverting the course of justice after lying about speeding points.
Ms Onasanya had been elected as a Labour MP but was expelled by the party the day after her conviction last December.
Mr Corbyn was a regular visitor to Peterborough in the build-up to the election, campaigning on several days.