British opposition leader Ed Miliband has launched his campaign to defeat Prime Minister David Cameron in upcoming general elections after primetime television interviews in which neither man landed a knockout blow.

Miliband, leader of the centre-left Labour, admitted on Friday that the May 7 poll would be the "tightest general election for a generation" during an event at the Olympic Park in east London.

Pledging to help the middle and lower income voters who Labour says have been left behind by Cameron's Conservative-led coalition government, Miliband insisted: "The Tories say this is as good as it gets - we say Britain can do better than this."

His speech came hours after he and Cameron took part in the first big television event of the campaign - a question and answer session in which they were grilled separately by veteran journalist Jeremy Paxman and a studio audience.

A Guardian/ICM snap poll after the program indicated that 54 per cent of viewers gave Cameron victory compared to 46 per cent for Miliband.

Commentators disagreed on whether Cameron, who focused on his party's economic record, or what one called a "surprisingly punk" Miliband came out on top.

Miliband, who struggles with an often awkward media persona, took a more combative approach to Paxman's questions. When asked if he had the steel needed to be prime minister, he responded: "Hell yes, I'm tough enough".

The Conservatives and Labour are neck-and-neck in opinion polls on 34 per cent, according to an average calculated by the UK Polling Report website.

The formal election campaign gets under way on Monday after parliament closed down on Thursday ahead of the election.

Miliband's speech featured pledges to protect Britain's state-run National Health Service (NHS) from what he claims is creeping privatisation under Cameron's coalition.

These included a new five per cent cap on profits for private companies which take on NHS contracts worth over 500,000 pounds.

Polling by IPSOS/Mori indicates that the NHS is the most important issue for voters at the election and Labour has put it at the centre of its campaign.