On immigration, Brown said Labor's points-based system is working to give the country the skills it needs. Cameron argued that immigration is "simply too high," with too much pressure on services, and that it must be brought down, with limits on immigrants from beyond the EU. Clegg also spoke out against Cameron's plan for a cap on the number of people coming into the country.

On political reform, Clegg accused the other leaders of saying much and doing little to clean up politics, calling it a "betrayal. " Brown said he agreed with the Lib Dem leader that reform is needed, saying the Tory idea of cutting the number of members of Parliament (MPs) will reduce representation for voters, and arguing the size of the House of Lords should be reduced by half.

But Cameron said Labor has had 13 years to sort out the House of Lords but not done it. Cutting MPs by 10 percent will cut costs and do a better job, he added.

On education, the prime minister said education budgets should not be cut. The Tory leader argued that discipline in schools must be improved. His Lib Dem rival also called for more freedom for staff, which will happen with "good, old-fashioned smaller class sizes."

On defense policy, Clegg said British troops are under-equipped and underpaid and that they should be the priority, rather than replacing the Trident nuclear deterrent. But Brown said the government is "doing the right thing by our troops" and is increasing spending on equipment, and he would not send troops into battle unprepared. Cameron said the Tories have had to fight a battle in Parliament to stop cuts to training for the Territorial Army.

The three leaders also clashed over spending cuts, tax and the reform of the National Health Service.

Sky news and the BBC will host the other two debates on successive Thursdays with foreign affairs and the economy topping the agenda separately.

According to Sky news, viewers so far think Brown is the candidate who is performing the worst. An instant poll showed that Brown has received 28 percent of vote, while Cameron and Clegg are neck and neck on 36 percent, by surveying more than 10,000 people via text message.

The current sample size of voters is 4,290.

Viewers are rating the three leaders in terms of their policies and personalities and voters' responses to comments on a range of issues are being tracked.

But some election experts said Clegg is still unknown to more than 1 in 3 voters, so he has a unique opportunity in the TV debates to insert his party into the nation's consciousness.

Britain's Prime Minister Gordon Brown (R), opposition Conservative Party leader David Cameron and Liberal Democrat leader Nick Clegg (L) greet each other after the first of Britain's leadership election debates at ITV studios with television moderator Alastair Stewart (2nd R) in Manchester April 15, 2010. [Xinhua]