United Nations Secretary General Kofi Annan has defended his organisation, insisting the world needs the UN.
Mr Annan says it is a decisive moment for the UN
In a speech in London, Mr Annan said the UN had an important role to play in fighting terror and poverty.
Earlier, UK Prime Minister Tony Blair strongly endorsed Mr Annan, calling him a "tremendous unifier".
The Secretary General has been under intense pressure over his leadership of the UN, following its role in the so-called oil-for-food scandal in Iraq.
Last year, a high-level panel established by Mr Annan proposed more than 100 measures to revitalise the organisation.
Mr Annan believes the UN stands at a crossroads.
In his speech in Whitehall, he admitted the UN needed to change to the way it operated.
But the world needs "a forum for collective decision-making and it needs an instrument of collective action", Mr Annan said.
"Our task is to adapt and update so that it can perform these functions in the 21st Century," he said.
Mr Annan, who has faced intense criticism of his leadership, particularly in the US, added: "Perhaps not everyone realises how much the UN is already moving with the times."
Mr Annan's agenda is ambitious, says the BBC's diplomatic correspondent Jonathan Marcus.
The Secretary General stressed that the time was gone when each country, or even continent, could look after its own security.
"We will not defeat terrorism unless we also tackle the causes of conflict and misgovernment in developing countries," he said.
"And we will not defeat poverty so long as trade and investment in any major part of the world are inhibited by fear of violence and instability," he added.
And it was time the world was united on the issue of Iraq, after last month's election, he said.
There was "an exciting moment of opportunity, in which the world can, and must, come together, whatever its past disagreements, to assist the Iraqi people," he said.
Plans for internal reform of the UN will be announced next month.