The rise of the Liberal Democrats has seen Brown and Cameron shift their focus, with both leaders launching stinging attacks on Clegg.

"An awful lot of fire was turned on Clegg, and I think they got him on the ropes, but not on the canvas," Jon Tonge, a politics professor at Liverpool University, said immediately afterwards.

Differing strategies

The leaders spent the early part of the debate arguing the best way to tackle the threat of international terrorism. Brown said that the problem needed a global response to ensure British security.

special report

"We've already got al-Qaeda in Somalia, we've already got problems with al-Qaeda in Yemen, we are having to take action with our multilateral partners to deal with these problems and will continue to have to do so," he said.

Cameron, who is seen as Brown's main rival for the prime minister's job, said that the UK should take a more holistic approach in dealing with the threat by making sure that foreign and domestic policy complement each other.

"I think we need to end the division between foreign policy and security policy and home office policy, bring it all together and think about our national security," he said.

Clegg, whose Liberal Democrat party was the only one of the three to oppose the invasion of Iraq, said that current strategy in the war in Afghanistan, where Britain has around 9,000 troops stationed, was failing.

"The problem is that we've done it in a manner where I don't think we've pursued the right strategy, we haven't given the right equipment to our troops, we haven't had proper international co-ordination on the ground in Afghanistan," he said.

Nuclear fallout

The leaders also disagreed over the future of the UK's nuclear weapons system, which Labour and the Conservatives want to update, but the Liberal Democrats want to replace with a cheaper alternative.

Key debate points
  War in Afghanistan
  UK nuclear weapons
  Climate change

"I don't think it's right to do what Gordon Brown and David Cameron want which is now to commit before even making a decision, to spend up to 100 billion pounds renewing in exactly the same old way the Trident nuclear missile system," Clegg said.

Britain's ambivalent relationship with Europe featured heavily in the debate.

Cameron believes that the UK's current relationship with the European Union has allowed Brussels too much influence in the country and says he would "stand up" for Britain if he became prime minister.

"I don't want us to join the euro, I want us to keep the pound as our currency," he said.

But Brown rejected what he called a "Britain-only solution" which could undermine the UK's move out of recession. 

"We're trying to get an economic recovery. That depends on Germany, France and other countries growing as well, taking our exports," he said.

Honours shared

Initial polls taken after the debate showed that both Cameron and Clegg had done well, with Brown in third place.

A YouGov poll for the Sun newspaper put Cameron in first place with 36 per cent, with Clegg in second on 32 per cent and Brown trailing with 29 per cent.

A ComRes poll for ITV news put Clegg first with 33 per cent, with Cameron and Brown tied for second place at 30 per cent.

Daniel Hamilton, a ComRes pollster, told Al Jazeera the debate had been close.

"It's really a score draw," he said. "The public are not really convinced by any of the three candidates. Fundamentally, it's a three horse race."

Current projections show the UK is heading for a hung parliament, where no clear winner emerges and parties are forced to attempt to form a governing coalition.