Defending the US-led invasion of Iraq, President George W Bush, on his first state visit to Britain, on Wednesday said the threat of terrorism is far from over.

He also praised the US-Britain alliance as 'very strong' and one based on values.

He also took a dig at United Nations by recollecting the failure of League of Nations to work against dictators and remembered the British victims of September 11, 2001 terror attacks on US.

"In some cases, the measured use of force is all that protects us from a chaotic world ruled by force," Bush told academics gathered at Whitehall Palace in an apparent defence of US invasion to dethrone Iraq's Saddam Hussein.

"There are principled objections to the use of force in every generation and I credit the motives behind these views," Bush said. "Those in authority are not judged only by good motivations. That duty sometimes requires the violent restraint of violent men."

About the terror attacks on US, he said, "The hope that danger has passed is comforting, is understanding, and it is false."

"These terrorists target the innocent and they killed by the thousands and they would, if they gain the weapons they seek, kill by the millions and not be finished... the evil is in plain sight. The danger only increases with denial," he said.

"More than an alliance of security and commerce, the British and American peoples have an alliance of values. And today this old and tested alliance is very strong," he said in his keynote speech at Banqueting House in London.

He stressed the common shared heritage between the two nations, which informed their values. "The fellowship of generations is the cause of common beliefs. We believe in open societies ordered by moral conviction."

"We believe in private markets humanised by compassionate government. We believe in economies that reward effort, communities that protect the weak, and the duty of nations to respect the rights and dignity of all," he said.

"Whether one learns these ideals in County Durham (Sedgefield -- Tony Blair's constituency) or in West Texas, they instill a mutual respect and they inspire common purpose," he added.

He said the foreign policies of Britain and the US were steered by their 'deepest beliefs' in the value of civil and human rights and in the dignity of every individual.

"The United States and Great Britain share a mission in the world beyond the balance of power or the simple pursuit of interest," he said. "We seek the advance of freedom and the peace that freedom brings."

"Together, our nations are standing and sacrificing for this high goal in a distant land at this very hour. America honours the idealism and bravery of the sons and daughters of Britain," he said.

Bush recalled the idealism of his predecessor Woodrow Wilson, the last US president to stay at Buckingham Palace, whose appeals for global justice in the wake of the First World War led to the creation of the League of Nations.

But in what appeared to be a dig at the United Nations for its refusal to back him in taking on Saddam Hussein, Bush recalled that the League had shown itself unable to deal with the threat posed little more than a decade after its creation by dictators like Adolf Hitler.

"The League of Nations, lacking both credibility and will, collapsed at the first challenge of the dictators," he said. "Free nations failed to recognise, much less confront, the aggressive evil in plain sight."

"Through World War and Cold War we learnt that idealism, if it is to do any good in this world, requires common purpose and national strength and moral courage and patience in difficult tasks. Now our generation have need of these qualities," he noted.

"The terrorists of September 11 had left the mark of murder on my country' and taken the lives of 67 British citizens," he said. It was understandable that, as time passed, people wanted to put that outrage behind them 'as if waking from a dark dream'.

Bush warned his audience that international terrorists would increase the severity of their attacks if they could obtain the necessary weaponry. Recent attacks across the world, including Baghdad and Istanbul, were 'art of a global campaign by terrorist networks to intimidate and demoralize all who oppose them', he added.

"These terrorists target the innocent and kill by thousands. Any they would, if they gain the weapons they seek, kill by the millions and not be finished. The greatest threat of our age is nuclear, chemical or biological weapons in the hands of terrorists and the dictators who aid them," he noted.

"The evil is in plain sight. The danger only increases with denial. Great responsibilities have fallen once again to the great democracies. We will face these threats with open eyes and we will defeat them," Bush declared.

The US leader argued that the peace and security of free nations now rested on three pillars. The first was that international organisations must be equal to the challenges facing the world.

"The United States and Great Britain have laboured hard to help make the United Nations what it is supposed to be an effective instrument of our collective security. The global danger of terror demands a global response," he cautioned.

He warned the credibility of the United Nations depended on its willingness to keep its word and act when required. The US and Great Britain would do everything they could to prevent the UN 'solemnly choosing its own irrelevance and inviting the fate of the League of Nations', he added.

Bush said it was not enough to meet the dangers of the world with resolutions. "We must meet those dangers with resolve," he said.

The NATO, he said, must also be willing to act beyond Europe. He added the world needed Europe and the US to work closely together. America was also cooperating with other nations in North Korea and Iran.

Bush said he understood objections to the use of force. But he said the second pillar of peace and security was the willingness of free nations to use force against aggressive states.

"Those in authority are not judged only by good motivations. The people have given us the duty to defend them. That duty sometimes requires the violent restraint of violent men. In some case the measured use of force is all that protect us form a chaotic world ruled by force," he concluded.

Earlier as his bulletproof Cadillac came into view on Whitehall, protesters began shouting: 'Go home Bush'. There were 100-150 protesters many carrying placards that said: 'Bush Blair butchers', 'world's number one terrorist' and 'Bush bullies Cuba'.

The road immediately outside the Banqueting House was closed to pedestrians while the president remained inside the building. Snipers armed with rifles monitored the crowd with binoculars from the roof of the building and a police helicopter circled above.

PTI