There was no clear winner in the only TV debate ahead of the British general election which pitted seven political leaders against one another.

Labour leader Ed Miliband topped one poll, the Scottish National Party's Nicola Sturgeon another and Mr Miliband tied with Prime Minister David Cameron and Ukip's Nigel Farage in first place in a third poll.

After Mr Cameron refused to face Mr Miliband in a head-to-head televised debate, the two-hour showdown broadcast on ITV was the only chance for the two men to confront one another on key election issues like the economy, jobs, immigration and health.

Mr Cameron accused the Labour leader of planning more debt, taxes, borrowing and spending and urged voters to let Conservatives complete their "long-term economic plan", telling them: "What my plan is about is basically one word - security. Security for you, for your family, for our country."

In a clear effort to bolster his credentials as an alternative premier, Mr Miliband repeatedly described what he would do "if I am prime minister".

He accused Mr Cameron of wanting to talk about the past rather than the future, and said: "Some people will tell you that this is as good as it gets for Britain. I say Britain can do so much better than it has done over the last five years."

Mr Farage repeatedly accused the other leaders of being "all the same" and said he was the only one who wanted to control immigration by pulling Britain out of the European Union.

However, he clashed with Plaid Cymru's Leanne Wood, who accused him of "scaremongering" and said he should be ashamed of himself after he raised the issue of foreign nationals receiving HIV treatment on the NHS and said: "We've got to put our own people first".

Nick Clegg sought to distance the Liberal Democrats from the two biggest parties, directly taking on Mr Cameron over what he termed "ideologically driven cuts" and challenging Mr Miliband to use the opportunity presented by the debate to apologise publicly for "crashing the economy" as part of the last Labour administration.

At one point, Mr Cameron was interrupted by a heckler from the 200-strong studio audience, Victoria Prosser, who demanded to be heard as she protested at the treatment of military veterans, shouting: "There's more of us than there is of them and they are not listening to us."

Mr Farage claimed an early advantage, with 24% of viewers polled at the half-way point by ComRes for ITV News rating him the best performer, ahead of Mr Miliband on 21% and Mr Cameron on 19%.

But by the end, the picture was less clear, with Mr Miliband leading an ICM poll for The Guardian with 25% of support, just ahead of Mr Cameron on 24%, with Mr Farage on 19%.

A ComRes post-debate poll for ITV News had Mr Miliband, Mr Cameron and Mr Farage tied in first place on 21%, with Ms Sturgeon on 20%.

A YouGov poll had the SNP leader top with 28% backing, followed by Mr Farage (20%), Mr Cameron (18%) and Mr Miliband (15%).

Mr Cameron enjoyed a strong lead in a key section of the ComRes poll, rated most capable of leading the country by 40% of the 1,120 viewers taking part, well ahead of Mr Miliband on 28%.

On immigration, Mr Farage said ordinary people had "paid a very high price" so corporate employers could benefit from cheap labour.

He said: "As members of the EU, what can we do to control immigration? Let me tell you - nothing."

But Mr Clegg said he would not "spread fear" about immigration and wanted Britain to remain "a decent, open-hearted, generous-minded nation who welcome people who want to come here and play by the rules".

Mr Cameron accused the Ukip leader of being "a back door to a Labour government" which would not deliver a referendum on Britain's EU membership.

Mr Miliband said Mr Cameron had marginalised Britain in Europe. He added: "I don't think our place lies outside the EU. I think that would be a disaster for jobs, a disaster for families and business."

In the most fiery exchange of the evening, Mr Farage was slapped down by Ms Wood after he said 60% of the 7,000 HIV diagnoses in the UK each year involved foreign nationals who "can come into Britain from anywhere in the world and get diagnosed with HIV and get the retro-viral drugs that cost up to £25,000 per year per patient".

"What we need to do is put the NHS there for British people and families, who in many cases have paid into the system for decades," said the Ukip leader.

Ms Wood responded: "This kind of scaremongering is dangerous, it divides communities and it creates stigma to people who are ill, and I think you ought to be ashamed of yourself."

Mr Farage replied: "Well, it's true. I'm sorry, we've got to put our own people first."