The three leaders campaigned in south-west England on Thursday
The second prime ministerial debate between Gordon Brown, Nick Clegg and David Cameron is underway in Bristol.
Mr Brown won the draw to deliver an opening statement, saying he offered substance over style and said the UK needed a "majority Labour government".
Mr Cameron said he offered "real change" rooted in family values and strong defence.
Mr Clegg said he would put the UK at the heart of Europe and the world, not complaining for the sidelines.
Last week's debate turned the campaign on its head with the Lib Dems jumping about 10% in many opinion polls - sparking fierce scrutiny of Mr Clegg.
Anti-war protesters were involved in a stand-off with police outside the venue in Bristol which is hosting the debate. Six people have been arrested for public order offences.
Ahead of the TV debate, Mr Clegg was attacked by various newspapers over party donations, past comments and his policies - his party said he was being smeared because of his success in the debate last week.
The Lib Dem leader dismissed the attacks and said he hoped people would not be "bullied into... not choosing something different".
He released copies of his bank statements and other paperwork in an attempt to clear up a row about a series of payments from party donors made directly into his private bank account.
The figures released by the party show donations from three private donors amounting to £19,690 were paid into Mr Clegg's account between January 2006 and January 2008.
But according to the Lib Dems' figures Mr Clegg paid £20,437.30 into party coffers between March 2006 and February 2008 for staffing costs.
A number of opinion polls placed the Lib Dems ahead of Labour and gaining on the Conservatives after last week's prime ministerial TV debate. Others, such as the YouGov/Sun survey of 20 April, have even put the Lib Dems in first place.
It has increased speculation that the general election on 6 May will result in a hung parliament - polls suggest the Conservatives are still slightly ahead but would not have enough seats to get an overall majority.
Mr Brown said he was fighting for a majority Labour government, not a coalition with the Lib Dems.
He said he would say "what I want to say and what I think needs to be said" in the second debate and told Sky News: "I plan to focus on what I think are the big challenges for the country and show that I've got the answers.
"Not everybody will want to see it that way but I want to talk about the substantive issues."
'Make a difference'
Mr Cameron said he was "looking forward" to the debate.
He said he had to explain to people how he would "make a difference".
Thursday's debate is being held at the Arnolfini Gallery in Bristol, hosted by Sky's political editor Adam Boulton, and is focusing on foreign affairs.
Mr Clegg will be in the middle of the three leaders, with Mr Cameron on the left when looking at the screen, and Mr Brown on the right, which is where he stood last week.
The Lib Dem leader is expected to highlight their opposition to the Iraq war. Labour and the Conservatives have already criticised the party's opposition to a "like-for-like" replacement for the Trident nuclear missile system.
Meanwhile the BBC Trust has rejected a joint appeal from the Scottish and Welsh nationalist parties against the BBC's decision to exclude them from the prime ministerial debates - separate leaders' debates are being held in Scotland, Wales and Northern Ireland.
Alex Neil, of the SNP, told the BBC that Scotland "wasn't mentioned once" in last week's debate adding: "Quite frankly, given the contribution that we make to the United Kingdom and the fact that we subsidise the United Kingdom, I think it's quite an insult that we were not represented in that debate."
And Nigel Farage, of the UK Independence Party, said all three leaders were "signed up 100%" for British membership of the EU, adding: "This is the one debate in which UKIP should have taken part. After all the last time we discussed this issue nationally was the European elections last year, we came second across the entire United Kingdom. So my feeling is, on these big questions tonight, no UKIP, no real debate."
• The second prime ministerial TV debate is being shown simultaneously on Sky News, Sky 3, and the BBC News Channel from 2000 BST to 2130 BST. It will also be streamed live on the BBC News website, and broadcast on BBC Radio 4. It will then be re-run on BBC Two, starting at 2330 BST.