Sir Keir Starmer said he took responsibility for Labour's "tough" local election results, adding: "Days like this don't weaken my resolve to deliver the change that I promised."
Early results saw Labour haemorrhage hundreds of councillors and eight local authorities across England while Reform, the Greens and Liberal Democrats all made gains.
Speaking at Kingsdown Methodist Church in Ealing, west London, on Friday morning, Sir said: "The results are tough, they are very tough, and there's no sugarcoating it.
"We have lost brilliant Labour representatives across the country, these are people who put so much into their communities, so much into our party.
"And that hurts, and it should hurt, and I take responsibility."
The Prime Minister faces further heavy losses as vote counting continues throughout Friday in both English local elections and contests for the Scottish Parliament and Welsh Senedd.
In Wales, Labour is expected to lose the national vote for the first time in more than a century while the SNP appears likely to remain the largest party in Scotland after 19 years in power.
Sir Keir has already faced speculation about his leadership, with the Times reporting Energy Secretary Ed Miliband had urged the Prime Minister to set out a timetable for his departure.
The Prime Minister went on: "When voters send a message like this we must reflect and we must respond.
"I think the vast majority of people do understand that we face huge challenges as a country.
"We've had a series of economic shocks in recent years and there's a very difficult international situation at present, they know that.
"But they still want their lives to improve, they still want to see the change that we promised, they know the status quo is letting them down and they're frustrated, they don't feel the changes."
Deputy Prime Minister David Lammy urged his party not to play "pass the parcel" with the leadership in response to the election results.
Labour sources pointed to poor local election results under previous prime ministers, including Sir Tony Blair who lost 1,100 councillors in 1999 but went on to win re-election in a landslide in 2001.
Meanwhile, Mr Farage hailed early results from Thursday's local election as a sign his party was on course for victory at a general election that is still up to three years away.
A jubilant Mr Farage heralded a "historic change in British politics," telling reporters "there is no more left-right" as his outfit was "scoring stunning percentages in traditional old Labour areas".
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