The general election race was thrown wide open today as polls showed that Nick Clegg's mould-breaking performance in the first of the televised leaders' debates had dramatically boosted his party's support, prompting the two major parties to vow to train their guns on its "eccentric, surprising" policies.

Labour and the Conservatives vied with one another to claim the other had been the bigger loser from the unprecedented debate, watched by 9.9 million viewers at its peak, as Labour's election co-ordinator, Douglas Alexander, admitted: "It is now impossible to predict the course of the next three weeks."

David Cameron conceded that Clegg had performed strongly, as his aides reviewed how they could puncture the Liberal Democrat leader's image as the anti-establishment outsider. Tory sources, ruing Cameron's personal decision to grant Clegg equal status in the three debates, started to point to his past as a full-time European commission bureaucrat, as well as his firm Europhile views.

There was further good news for the Lib Dems when a YouGov poll for the Sun put them in second place on 30%, pushing Labour into third on 28%, with the Conservatives in the lead on 33%. Labour have not trailed in third place in a national poll of this kind since the days of the SDP/Liberal Alliance in the 1980s.

Some Conservatives are pressing Cameron to expose Clegg's support for an earned amnesty for illegal immigrants, his willingness to keep anyone given a sentence of less than six months out of jail, and the credibility of the party's claim that it can raise £4.8bn a year from anti-tax avoidance measures.

Labour, happy to see a Liberal Democrat wave so long as it does not convert into a tsunami, continued to emphasise the similarities between Lib Dem and Labour policies. Lord Adonis, the transport secretary and a former Lib Dem, took hints of postelection co-operation a stage further by saying he would not tell voters whether to back Labour in Lib Dem-Tory marginals.

In the first poll on the impact of the debate on the state of the parties, a ComRes survey for ITN found the Lib Dems at 24%, up 3% on a similar poll two days earlier, with Labour down 1% on 28% and the Conservatives unchanged on 35%.

An ICM poll for the Guardian found that 23% of voters who watched the debate said they would change their vote, with most going to the Lib Dems.

The Sun/YouGov poll conducted after the TV debate saw the Lib Dems take four points from the Conservatives, three from Labour and one from other parties in the poll. A Harris poll for the Daily Mail put the Lib Dems level with the Conservatives on 32% and Labour on 26%.

Labour believes a strengthening Lib Dem poll rating can deprive the Tories of an overall majority by putting some vulnerable Labour seats out of their reach.

Labour plans to focus on the economy next week with a slew of statistics due to be published, culminating on Friday with the figures showing whether the economy continued to grow in the first quarter.

Brown will have advance warning of the figures before he takes part in the second leaders' debate on Thursday night.

Alexander highlighted the extraordinary failure of Cameron not to use the TV debate to showcase his call to roll back the state and roll forward society. "Within two days of his manifesto launch, his own big idea – the Big Society – itself became the Great Ignored, another strategy dumped as quickly as it was born."

Clegg told party aides that they now faced the most difficult week of the campaign as the media scrutiny grows. Insisting his feet were "firmly on the ground", he said: "Some people are getting a bit hyped up about this."

With some Lib Dems describing Clegg as a potential British Barack Obama, the former party leader Lord Ashdown vowed: "No triumphalism, this is serious stuff now – they are going to go for us, you watch."

Cameron said he completely accepted that Clegg had "a very good debate". But he pointed out: "A plague on both your houses is a great tune to play in politics. I always knew that if you do debates you are going to give them a massive platform but I always thought it was worthwhile. We all know that polls react to news cycles. It is a very depressing thing about politics: you do your manifesto launch or conference speech and you get a bounce and you think, 'Fantastic we are on our way' – and then a few days later you think: 'Hold on, what did we do all that for?' "

Reflecting an intense debate inside the Tory party on how to handle the Lib Dem intruder, the shadow schools secretary, Michael Gove, vowed that they now intended to subject the Lib Dems' policy programme to intense scrutiny. He emphasised Lib Dem plans to join the euro, scrap Trident and offer an amnesty to illegal immigrants, which, he claimed, lay outside the political mainstream.

"The greater degree of scrutiny these policies have, the more that people will realise that while Nick Clegg is a very attractive individual in many ways, the policies of his party are outside the mainstream and a little bit eccentric – not necessarily what you would want at a time of crisis and difficulty."

The Tories will today move to win back voters tempted to support the Lib Dems by pledging to offer all of Britain's 5 million public sector workers the right to request flexible working. At the moment this is only available to parents of children under the age of 16.

In an article for the Guardian's Comment is free, the shadow chancellor, George Osborne, writes: "This radical step will help more government employees to achieve a better work-life balance and gain more control over their lives. As we've seen with companies like BT and Asda, where the majority of employees are now working flexibly, this will particularly benefit women, older people and employees with health conditions."