Manchester - Britain's incoming prime minister Gordon Brown on Sunday vowed to learn the lessons of Tony Blair's "divisive" decision to go to war in Iraq, as he was finally confirmed leader of the Labour Party.

But while both Brown and his new deputy in the governing party acknowledged that mistakes had been made, he offered few signs that his Middle East policy would immediately differ from that of Blair when he takes over on Wednesday.

In his acceptance speech after being elected unopposed as Labour leader, Brown pledged to forge "the strongest multilateral response" to tackling extremism and other issues such as the fight against climate change.

"In Iraq, which all of us accept has been a divisive issue for our party and our country, in Afghanistan and in the Middle East, we will meet our international obligations," he told Labour members here.

"We will learn lessons that need to be learned."

As anti-war protesters outside called for him to withdraw British troops from Iraq immediately, Brown gave no hint of a different approach, even though Blair's stance on Iraq arguably hastened his departure from office.

Echoing Blair's philosophy in international affairs, he recognised that defeating extremism went further than the sole use of military force.

"It is also a struggle of ideas and ideals that in the coming years will be waged and won for hearts and minds here at home and round the world," Brown said.

For her part, Labour's new deputy leader Harriet Harman, currently a justice minister, promised a more open and accountable government in a veiled criticism of Blair's leadership.

"You want our plans to be debated and presented to parliament, not briefed and spun to the media," she said.

"You want us to acknowledge the anger and division caused by Iraq, and we do."

Brown -- Blair's finance minister since he became prime minister in 1997 -- was confirmed Labour leader after neither of the two prospective candidates who hoped to run against him could get enough support.

He becomes premier as head of the largest party in parliament. Governing parties in Britain can change leader mid-term without having to call a general election.

Menzies Campbell, leader of the opposition Liberal Democrats, said Brown should call an early general election to give him a mandate to govern the country, describing the recent weeks as "the longest coronation in history."

For his part, Brown quickly named an election supremo, current Transport Secretary Douglas Alexander, who will be responsible for ensuring Labour will be ready to go to the polls whenever Brown decides.

A general election is not due until May 2010.

Brown said he took the job with "humility, pride and a great sense of duty" and recognised it as an "awesome responsibility".

"This week marks a new start, a chance to renew. And I say to the people of Britain: the new government I will lead belongs to you. I will work hard for you. I shall always try my utmost. I am ready to serve," he said.

Brown also offered little sign of diverging radically from Blair's modernising, centre-left "New Labour" agenda, signalling a change in style rather than substance.

But he recognised that change was needed if Labour was to secure a fourth straight general election win no later than 2010, which included re-engaging with a disaffected and mistrustful electorate to tackle future challenges.

"The party I lead must have more than a set of policies -- we must have a soul," he added.

Brown indicated his credo would be based on his belief in "duty, honesty, hard work, family and respect for others", increasing opportunity for all and cutting social injustice and inequality.

"They guide my work, they are my moral compass. This is who I am," said Brown, describing himself as a "conviction politican".

Introducing Brown to a standing ovation, Blair put aside the troubles that have often characterised their relationship, particularly over when he would hand over power, offering a ringing endorsement of his successor.

"He's a man who's strong and sound in his convictions, true to his principles, a man with every quality to mark him out as a great prime minister of our country," he said.

"I think from his character that he will give of his best in the service of our country and I know from his record as Chancellor (of the Exchequer -- finance minister) that his best is as good as it gets."