U.S. President Barack Obama sought to reassure the world Wednesday that American and European influence remains as dominant as ever, even as rising powers like China and India assert themselves.

To the British Parliament seated at majestic Westminster Hall, Obama declared: "The time for our leadership is now."

"Even as more nations take on the responsibilities of global leadership," Obama said, "our alliance will remain indispensible to the goal of a century that is more peaceful, more prosperous and more just."

"After a difficult decade that began with war and ended in recession, our nations have arrived at a pivotal moment once more," he said.

Obama was granted the honour of being the first U.S. president to speak from the grand setting of Westminster Hall, and he received a deeply friendly welcome. He recounted a history between two countries an ocean apart that began in war but grew into an indispensible global force for economic growth, security, democracy and peace.

His speech came not long after Obama joined Prime Minister David Cameron in promising jointly to continue a relentless and punishing campaign against Moammar Gadhafi's forces in Libya, saying there "will not be a let-up" in pressure to force Gadhafi out.

In his appearance before Parliament, Obama talked glowingly about a historically strong partnership with Great Britain.

"The path has never been perfect," he said. "But through the struggles of slaves and immigrants; women and ethnic minorities; former colonies and persecuted religions, we have learned better than most that the longing for freedom and human dignity is not English or American or Western — it is universal."

Obama spoke to both houses of Parliament and British leaders present and past who were gathered in the cavernous 11th-century hall where generations of rulers have held coronation banquets and where many others lay in state while awaiting burial.

Obama welcomes the Queen to a dinner at Winfield House, the residence of the ambassador of the U.S., in Regent's Park, London, as part of the president's state visit to the U.K. (Yui Mok/PA Pool/Associated Press)

His address came midway through a four-country European tour during which he's connected with his unlikely Irish roots and enjoyed the hospitality of Queen Elizabeth even while keeping an eye on events at home where casualties mount from a monster tornado in Missouri.

Wednesday's speech was billed as the centrepiece of the president's tour, and he addressed grave questions of war, peace and economic strain, calling on Britain and the U.S. to meet the challenges together, and more broadly on the world to move toward democracy and universal rights.

"Our idealism is rooted in the realities of history — that repression offers only the false promise of stability; that societies are more successful when their citizens are free; and that democracies are the closest allies we have," the president said.

On foreign soil, Obama also sprinkled his speech with promotions for his political agenda at home. He called for spending on education and science even during austere times; for more international progress on reducing the carbon emissions that cause global warming; and for a government assurance that people can get health care.

Here, too, he said the United States and Britain can serve as models for emerging giants such as China, India and Brazil.

"The successes and failures of our own past can serve as an example for emerging economies that it's possible to grow without polluting; that lasting prosperity comes not from what a nation consumes, but from what it produces, and from the investments it makes in its people and infrastructure," he said.

The president received a sustained, standing ovation for his defence of the American-British alliance and how the two "enduring allies" will be a defining force for good in the future, as well.

"As two of the most powerful nations in history, we must always remember that the true source of our influence hasn't just been the size of our economy, the reach of our military, or the land that we've claimed," the president said. "It has been the values that we must never waver in defending around the world the idea that all human beings are endowed with certain rights that cannot be denied."

As revolutions sweep the Middle East and North Africa and NATO forces bombard Libya, Obama gave a robust defence of the need for the U.S. and Britain to put their military might at the service of people around the world seeking freedom.

He defended the war in Afghanistan, where Britain has 10,000 troops committed and there's keen interest in U.S. plans for the withdrawal of forces, and the action in Libya, where he said intervention prevented a massacre.

"If we fail to meet that responsibility, who would take our place?" the president asked. "Our action, our leadership, is essential to the cause of human dignity. And so we must act and lead with confidence in our ideals, and an abiding faith in the character of our people, who sent us here today."