Sir Keir Starmer has said the "opportunity for change" will define the general election on 4 July.
The Labour leader said Rishi Sunak's announcement was the "moment the country needs and has been waiting for".
Speaking in central London, Sir Keir reiterated the six "first steps" for government he outlined last week and added: "It will feel like a long campaign... but no matter what else is said and done, that opportunity for change is what this election is about."
He declared: "It is time for change."
His words came moments after Mr Sunak ended months of speculation about the timing of the general election and confirmed it would take place on 4 July.
In a statement outside Downing Street in the pouring rain, the prime minister said he had met with the King to request the dissolution of parliament.
"The King has granted this request and we will have a general election on the 4th of July", he said.
The prime minister took aim at Sir Keir - who, according to polls, is on course to succeed him in Downing Street - saying he has shown "time and time again that he will take the easy way out and do anything to get power".
Referring to the now-abandoned pledges Sir Keir made to Labour members in his bid to succeed Jeremy Corbyn as leader, Mr Sunak said: "If he was happy to abandon all the promises he made to become Labour leader once he got the job, how can you know that he won't do exactly the same thing if he were to become prime minister?"
It comes after official figures showed inflation had come down to 2.3% in April, with Mr Sunak's decision to call and election for next month suggesting he hopes to capitalise on the improvement in the economic landscape.
He said the reduction in inflation was "proof that the plan and priorities I set out are working".
However, he said "this hard earned economic stability was only ever meant to be the beginning", and asked the public: "The question now is how and who do you trust to turn that foundation into a secure future for you, your family and our country?
"Now is the moment for Britain to choose its future and to decide whether we want to build on the progress we have made or risk going back to square one. With no plan and no certainty."
Ed Davey, the leader of the Liberal Democrats, responded positively to the election announcement and urged voters to "kick this appalling Conservative government out of office".
"For years the Conservative Party has taken voters for granted and lurched from crisis to crisis while the problems facing the country are getting so much worse," he said.
"The NHS has been brought to its knees, people's mortgages and rents have soared by hundreds of pounds a month, and water companies have got away with pumping filthy sewage into our rivers and beaches.
"All because this Conservative government is more interested in fighting between themselves than standing up for the needs of the country."
A party spokesperson said Mr Davey was "on his way now to Michael Gove's true blue Surrey seat" - a reference to the threat the Lib Dems pose in the Tories traditional heartlands in southern England.
John Swinney, the newly appointed leader of the SNP, said: "I look forward to leading the SNP in this election.
"This is the moment to remove the Tory government and put Scotland first by voting SNP. People in Scotland know we stand up for them and protect them from the damage done by Westminster."
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