Tony Blair has defended his tough stance on Iraq, warning that failing to topple Saddam Hussein will have grave consequences.

Speaking as a mass rally against a war got underway in London, the prime minister said that Iraq had to be disarmed.

Going further than ever before, Blair said he wanted regime change in Iraq. Ridding the world of Saddam, he said, would be an "act of humanity".

But in an impassioned address he signalled that Britain was prepared to give weapons inspectors more time.

Warning the UN that it could not hold back for ever, Blair said he still hoped a peaceful solution could be found.

"At every stage, we should seek to avoid war. But if the threat cannot be removed peacefully, please let us not fall for the delusion that it can be safely ignored," he told delegates.

"If we do not confront these twin menaces of rogue states with Weapons of Mass Destruction and terrorism, they will not disappear. They will just feed and grow on our weakness."

And he warned that indefinitely delaying a military offensive would serve no purpose.

The prime minister said Saddam would never hand over his weapons arsenal.

"No-one seriously believes he is yet co-operating fully. In all honesty, most people don't really believe he ever will," he said.

"So what holds people back? What brings thousands of people out in protests across the world? And let's not pretend, not really that in March or April or May or June, people will feel different.

"It's not really an issue of timing or 200 inspectors versus 100. It is a right and entirely understandable hatred of war. It is moral purpose, and I respect that."

Making the "moral case" for a war, Blair warned of a "bloody conflict" if Iraq was not dealt with through the United Nations.

"We will come through it by holding firm to what we believe in. One such belief is in the United Nations," he said.

"I continue to want to solve the issue of Iraq and weapons of mass destruction through the UN. That is why last November we insisted on putting UN inspectors back into Iraq to disarm it.

"To anyone familiar with Saddam's tactics of deception and evasion, there is a weary sense of déjà vu. As ever, at the last minute, concessions are made.

"And as ever, it is the long finger that is directing them. The concessions are suspect. Unfortunately the weapons are real."

Whilst he warned that he did not court unpopularity as "a badge of honour" Blair said he would take the decisions he believed to be right.

Addressing the "Stop the War" protesters, he said: "There will be no march for the victims of Saddam, no protests about the thousands of children that die needlessly every year under his rule, no righteous anger over the torture chambers which if he is left in power, will be left in being.

"I rejoice that we live in a country where peaceful protest is a natural part of our democratic process.

"But I ask the marchers to understand this. I do not seek unpopularity as a badge of honour. But sometimes it is the price of leadership. And the cost of conviction.

"But as you watch your TV pictures of the march, ponder this:

"If there are 500,000 on that march, that is still less than the number of people whose deaths Saddam has been responsible for.

"If there are one million, that is still less than the number of people who died in the wars he started."