With bags under his eyes and a croaky voice after 36 hours of non-stop campaigning up and down Britain, Cameron summoned up one last burst of energy to exhort voters to throw out the Labour Party in Thursday's election.

"If that isn't a cause worth staying up all night for, I don't know what is," Cameron told a small but noisy crowd of supporters gathered in a tight huddle around him in a cavernous railway hangar.

The evening had begun in a subdued fashion, with Cameron giving a live interview on the BBC's flagship evening news program while all around him supporters in party t-shirts and armed with campaign placards were asked to remain silent.

The scene was in keeping with the election campaign, which has been dominated by televised events, particularly three debates between Cameron, Labour's Prime Minister Gordon Brown and Liberal Democrat leader Nick Clegg.

But Cameron, a confident public speaker, is often at his best when interacting directly with voters and after he was freed of the TV earpiece he gave the Bristol crowd the call to arms they had been waiting for.

"When there are a million young people unemployed and our economy's stuck in a rut, what do we need?" he asked, drawing a cry of "Change!" "When the NHS (National Health Service) and the schools are drowning in bureaucracy, what do we need?" he asked.

"Change!" roared the crowd again.

He continued in that vein, listing what he said were the greatest ills afflicting Britain and eliciting the repeated call for change from an audience now ready to eat out of his hand.

Perhaps mindful of a 1992 blunder by then Labour leader Neil Kinnock, who arrived triumphantly by helicopter at a huge final campaign rally only to lose an election he was tipped to win, the Conservatives kept the Bristol event small.

Polls nevertheless suggest no party will win an outright majority and the quirks of the British electoral system mean the ruling Labour Party could finish third in terms of vote share yet remain the largest bloc in parliament.

"It's the last event of the campaign and I wanted to say that I was there, just before the election," said Harriet Maltby, 20, a politics student at Oxford University sporting a blue party t-shirt and a sparkly skirt emblazoned with the British Union Jack flag.

Mel Tanner, a lifelong Conservative who had been busy helping out with the campaign in Bristol, was impressed with Cameron but in no doubt about the challenges ahead if he won office at a time of economic woes and mounting public debt.

"Given the circumstances it's going to be jolly hard, but he's totally dedicated, he believes completely in what he says and he wants the best for Britain," she said.