Libl Dem leader fears that if Labour or Conservatives tried to impose cuts after narrow win it 'could spark Greek-style unrest'

Britain will be hit by waves of "Greek-style unrest" if a Tory or Labour government narrowly wins the election and then tries to push through draconian spending cuts, Nick Clegg warns today.

In an interview with the Observer, the Liberal Democrat leader says he fears "serious social strife" would break out on the streets if a government with limited support at the election on 6 May then raised taxes, laid off public-sector workers and froze wages.

As the main parties prepared to launch their election manifestos early next week, Clegg said it was "stating the obvious" to say that a hung parliament, in which the main parties were forced to work together, would be good for the country. The alternative would be to have a government that lacked support across huge parts of Britain at a time when emergency measures were needed to cut the deficit.

Advancing his case for a fairer voting system that would return MPs in proportion to the number of votes cast, he said that Labour won office in 2005 with the support of 22% of eligible voters.

"Imagine the Conservatives go home and get an absolute majority, on 25% of the eligible votes," Clegg said. "They then turn around in the next week or two and say we're going to chuck up VAT to 20%, we're going to start cutting teachers, cutting police and the wage bill in the public sector. I think if you're not careful in that situation… you'd get Greek-style unrest. And so my warning to people who think the old politics still works, is be careful for what you wish for."

In his own constituency city of Sheffield, where there are no Conservative MPs or councillors, and a high proportion of public-sector workers, he fears a ferocious backlash against potential Tory cuts.

"Suddenly these people will be told by a government that has no legitimacy [in the area] in their eyes that this government is going to slash and burn, having promised them something else."

Clegg said he was "not campaigning for a hung parliament" but it would be preferable to rule by a party with a tiny majority based on a minority of votes. "Do I think politicians working together can be a good thing? Of course it can."

Last month strikes and protests against the Greek government's harsh austerity measures caused chaos, with 10,000 marching in protest in Athens.

A series of opinion polls last night suggested that a hung parliament, or a small Tory majority, remained the most likely outcomes.

A survey of 96 Labour-held marginals for the News of the World found Conservative support had fallen by four points to 36% since January, while the Lib Dems had surged five points to 19%. Labour remained on 37%. The figures would be likely to leave Mr Cameron in control of 308 seats – not enough to wield a majority. Labour would have 248 seats and the Lib Dems 61.

Meanwhile, a YouGov survey for the Sunday Times found the Tory lead had dropped from 10 to eight points over the past week. It put the Tories on 40%, Labour (up three) on 32% and the Lib Dems down two on 18%.

Clegg's comments come as both Labour and the Liberal Democrats slammed Tory proposals to give married couples a tax break.

George Osborne, the shadow chancellor, laid out details of the £550m plan yesterday, revealing it would benefit only one-third of Britain's 12.3 million married couples – those where there is one partner on a low to middle income and the other does not earn. They would gain £150 a year.

Clegg, campaigning with his wife Miriam Gonzalez Durantez in Sheffield, described them as a "throwback to the Edwardian era", while Gordon Brown accused the Tories of taking money away from struggling families, claiming they would make heavy cuts in child tax credits, child trust funds, Sure Start and schools.

Yvette Cooper, the work and pensions secretary, said: "It is unfair to tell children whose parents, for whatever reason, aren't married that they and their families are second class. What kind of signal is that?"

Speaking during a visit to Cheam, in south London, Tory leader David Cameron said: "We should have a society where we back commitment and where we recognise marriage and civil partnerships in the tax system. Would it be a good thing if more people came together and stayed together and showed commitment? I think it would."

The Observer has learned that Labour's manifesto, to be unveiled on Monday, will contain plans to force energy companies to reduce the bills of around a million poorer pensioners by £100 a year. Companies will have to offer reduced tariffs to all over-75s who are on income support. Labour's manifesto will focus on rebuilding the economy, reforming public services and rebuilding trust in politics through measures that will include reducing the voting age to 16.

Campaigning in his Kirkaldy and Cowdenbeath constituency, Brown said Labour's offer to voters would be credible rather than exciting. "It is about substance in the end," he said.