Labour was bracing itself for a difficult night as the first results were declared from "Super Thursday" elections across the UK.
As senior Labour figures sought to put a brave face on the expected loss of seats in the Scottish Parliament and English councils, one backbencher warned the party was "moving away from Government" under Jeremy Corbyn's leadership.
And shadow cabinet minister Andy Burnham revealed he is considering running for mayor of Greater Manchester, in an apparent sign that pessimism about Labour's prospects of regaining power at Westminster reaches into the party's highest echelons.
With 45 million people eligible to vote, "Super Thursday" was being seen as the first nationwide verdict on Mr Corbyn since he stormed to victory with massive grassroots support as Labour leader last September.
Results were expected overnight in elections to the Scottish Parliament, Welsh Assembly, dozens of English councils, the Salford mayoralty and parliamentary by-elections in the safe Labour seats of Sheffield Brightside and Ogmore in south Wales.
Voters in Scotland were expected to give Labour a bloody nose, with The Sun newspaper predicting that Conservatives would "comfortably" push them into third place north of the border.
Counting for the Northern Ireland Assembly and the mayor of Liverpool will not begin until the morning and voters in London will have to wait until Friday evening to learn the identity of their new mayor and Assembly members.
Also being announced on Friday were the Police and Crime Commissioners for 36 English force areas, while counting in some contests - including the Bristol mayoralty - will continue into the weekend, finally ending with the declaration of four Welsh PCCs and councillors in Bristol on Sunday.
On the eve of polling, Labour retreated from comments its leader made in the final days of the campaign that the party was on course for gains in the local authority elections.
Shadow chancellor John McDonnell said the party's "objective" was to narrow the 6.3% advantage enjoyed by the Conservatives in last year's general election, before making "steady progress" towards victory in 2020.
In the night's first result, Labour consolidated its hold on its stronghold Sunderland Council, gaining a single seat from an independent.
But Bermondsey & Old Southwark MP Neil Coyle told BBC2's Newsnight that the party should be gaining seats across the country at this stage in the Parliament in order to have a chance of returning to power at the general election.
Urging the leader to broaden the political "diversity" of his inner circle, Mr Coyle said: "We are moving further away from Government, I think, because we seem to be fixated on some issues that are peripheral and we seem to have a team which isn't projecting either unity within the party or a vision and policies that the voters want."
Deputy leader Tom Watson urged Labour activists and MPs to "respect the mandate" given to Mr Corbyn by party members and supporters last year, and suggested that it was too early in his tenure to expect him to be chalking up significant electoral advances.
"I think most people would recognise you can't consolidate your position in only eight months," Mr Watson told Sky.
An upbeat Mr Corbyn said he was feeling "very happy" and gave a thumbs-up as he went to put his cross in the box at a polling station in Islington.
The party's main hope of success is that Sadiq Khan takes back City Hall in London after eight years of Tory rule.
Conservative mayoral candidate Zac Goldsmith has tried to seize on the anti-Semitism controversy engulfing Labour in a bid to boost his chances after trailing in the polls.
But shadow cabinet member Chris Bryant hit back at David Cameron's exploitation of the issue in the run-up to polling day, telling the House of Commons that there was "no gutter low enough for the Prime Minister to slop around in".
Responding to Mr Cameron's call on Wednesday for Mr Corbyn to withdraw his earlier description of Hamas and Hezbollah as "friends", Mr Bryant said: "That kind of despicable smearing of one's opponents I think degrades the whole of politics.
"I would just say gently to the Government that those who live by the gutter die in the gutter."
In Scotland, Nicola Sturgeon is aiming to lead the SNP to a historic third term, while Ukip is hoping for a breakthrough in Welsh National Assembly elections.
In all, 2,700 seats are being contested in 124 English councils.