The leaders of the three main political parties are preparing to hold their first TV debate on Thursday evening.

The live broadcast will see Gordon Brown, David Cameron and Nick Clegg answer questions chosen by a 200-strong audience, and from viewers' e-mails.

Being filmed at Granada Studios in Manchester, the focus for the 90-minute programme is domestic affairs, but leaders can air other issues.

It is being shown on ITV 1. Sky and BBC 1 will broadcast the other two debates.

The US-style televised debates between the three party leaders are the first of their kind in the UK.

'Huge job interview'

Gordon Brown told ITV 1 "he was not nervous, but determined to get our views across".

"You gotta be realistic and honest that nothing will be the perfection you want it to be, but I'm determined to get my message across," he said.

Asked why he had agreed to do it, Mr Brown said: "I think the public in this television age have got a right to see the different parties' prospectuses through their leaders, and I think people will be able to judge whether our policies are better."

Conservative leader David Cameron said he was "looking forward to it".

"It is an historic first for our country to have live televised debates between the people putting themselves forward to be prime minister.

"It is a great opportunity to try to communicate to those millions of people in our country who are switched off politics."

Lib Dem leader Nick Clegg said he "wouldn't be human" if he wasn't excited about the live debates.

"It's like having one big huge job interview in front of the whole nation," he said. "But the main thing is I know myself well enough, I've actually got to enjoy it.

"That's what I want to do, instead of constantly worrying about saying this or that or getting that fact wrong or right."

Rehearsals

BBC political editor Nick Robinson said both Labour and the Conservatives were resorting to mind games, playing down how well their leaders are likely to perform, each saying that the other will have the better debate.

Yet he added all three leaders had been hard at work practicing for the big night.

For Mr Cameron's rehearsals, he said shadow immigration minister Damian Green had been playing the role of Mr Brown; while the prime minister had enlisted Alistair Campbell, Tony Blair's former director of communications, to play the Tory leader.

"But they know that when the big moment comes they are on their own," said our political editor.

He added that it was also a big night for Mr Clegg, and a question of whether he has just a "walk on part", or if he can show he should be treated as someone ready to claim his place in high office.

The debates are bound by more than 70 rules agreed after weeks of negotiations between senior Labour, Conservative and Liberal Democrat officials, and the broadcasters.

Before the questions begin, Mr Brown, Mr Cameron, and Mr Clegg will be allowed to make a one-minute opening address, and a 90-minute closing statement at the end of the show.

The audiences will be asked to applaud at the start and end of the programmes, and will not be allowed to make any response to the answers or comments given by the three leaders.

They will also not be allowed to ask their questions directly, instead they will be presented on their behalf by the presenter of each show, which in ITV 1's case is Alastair Stewart.

Mr Brown, Mr Cameron and Mr Clegg will have a time limit enforced on their answers.

With the three only set to face eight questions on Thursday night, Mr Cameron said he was concerned the debate risked being "slow and sluggish".

Most of the 200-strong audience in each debate will be picked by pollsters ICM from the local area to ensure a balance of gender, age, ethnicity, social class and voting intention.

Broadcasters drew lots on the order in which debates will be screened and the themes.

Sky News will be showing the second debate on Thursday, 22 April, which will look at foreign affairs. The final debate on BBC 1 on Thursday, 29 April will focus on the economy.

The first debate follows after two opinion polls said the Conservatives had seen an increase in support.

The YouGov daily tracker poll for The Sun said on Wednesday night that the Conservatives had extended their lead over Labour to nine points.

It puts the Tories on 41%, up two points on the previous day, with Labour up one on 32%, and the Lib Dems down two on 18%.

A separate poll for the Daily Telegraph said the Conservatives were leading Labour in 100 key marginal constituencies. It put the Tories on 43%, with Labour on 31%, and the Lib Dems on 20%.

• The First Election Debate programme will be shown on ITV 1 on Thursday, 15 April, starting at 2030 BST. In Scotland it will be shown on STV, and on UTV in Northern Ireland.