The three leaders returned to the campaign trail on Friday

Gordon Brown and David Cameron have conceded Lib Dem rival Nick Clegg did well in the UK's first prime ministerial TV debate.

The Tory leader said Mr Clegg, who topped most instant polls, had a "good debate" while Mr Brown said he would be "rightly pleased with his performance".

Both said they enjoyed the debate which was watched by 9.9m people at its peak.

All three are back campaigning on Friday, Mr Brown in Sussex, Mr Clegg in Warrington and Mr Cameron in Nantwich.

The Conservative leader was joined by Take That's Gary Barlow at a high school, as he launched plans for a national "School Stars" singing competition, Mr Brown was joined by comedian Eddie Izzard in Hove while Mr Clegg was visiting Warrington Rugby Club.

In other election developments on Friday:

The three leaders clashed over issues including tax, immigration and expenses in the 90-minute debate on ITV1, which early overnight figures suggest had an average audience of 9.4m, peaking at 9.9m - compared with 8m for Coronation Street.

Two more debates will be broadcast by Sky and the BBC over the next two weeks, in the run-up to the general election on 6 May.

'Hard-fought campaign'

Two opinion polls taken immediately after the debate, by YouGov and Populus respectively, suggested Mr Clegg won.

And an ITV/ComRes poll conducted after the debate suggested national support for the Liberal Democrats was up 3% to 24%, compared with two days ago, leaving Labour on 28% and the Conservatives in front on 35%.

It was based on a telephone poll of a selected group of 4,032 viewers, taken immediately after the debate, the results of which were adjusted to make them nationally representative.

Asked about Mr Clegg's performance in a morning interview with BBC Manchester, Mr Cameron said: "I think he had a good debate but I must say I just enjoyed being able to talk to people at home, to address the questions that I think are the big questions at this election like immigration and the economy and crime."

'Bit eccentric'

He said he faced a "huge challenge" but added: "We are in the middle of a hard-fought and tough election campaign. It's a campaign I believe we can win. It's a campaign I believe we ought to win because the country is crying out for change."

His colleague Michael Gove told Sky News Mr Clegg could "play the role of the sparky and feisty outsider" but said some Lib Dem policies were "outside the mainstream and a little bit eccentric".

Mr Brown, in an interview with BBC Radio Sussex, said Mr Clegg, who as leader of the UK's third biggest party does not get as much exposure as the leaders of the two biggest parties, had been "introducing himself in many ways to the public, in a mass way, for the first time".

"I think he'll be rightly pleased with his performance. I think at the end of the day... when all the dust settles, when people start sort of examining what are the policies, I don't think people know much about Liberal policies, and I think they do know more about our policies.

He said he had "enjoyed" the debate, adding: "I think it's part of the debate we should have in Britain. I think it's energised the campaign."

'Three-horse race'

Addressing party activists in Warrington on Friday, Mr Clegg said it had been an "important moment in this campaign" but told them it was "just the start" and urged them to get out campaigning.

One of his predecessors as Lib Dem leader, Lord Ashdown, said the debate was "potentially a game changer".

But he warned: "There's a long way to go yet. There's three weeks, there's two more debates, and now that we've made this a three-horse, rather than a two-horse race, they're going to be coming for us, we've got to be ready for that."

BBC political editor Nick Robinson said the emergence of Mr Clegg as a serious player would be the most significant development - but there was a question mark over whether instant polls suggesting he won, could be converted into more support for his party.

He added that, amid predictions of a hung parliament in which Mr Clegg's party could hold the balance of power, the Lib Dems would be wooed, attacked and scrutinised with renewed vigour.

The Scottish National Party and Plaid Cymru are not taking part in the debates, although the BBC is to hold separate leaders' debates in Scotland, Wales and Northern Ireland.

SNP Leader Alex Salmond told the BBC: "You had three Westminster politicians who agree with each other on 99% of issues, and therefore the debate couldn't really come alive because of that convergence on the things that matter, and also, of course, because the audience weren't allowed to really participate."

Plaid Cymru leader Ieuan Wyn Jones said: "In what was a very sterile debate, not once did we hear the word Wales mentioned by any of the leaders. Indeed much of what they said was irrelevant to our communities."

UK Independence Party MEP Nigel Farage, said there was not a "single memorable phrase" in the entire 90 minutes.

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