PRAGUE — President Obama vowed yesterday to pursue the elimination of nuclear weapons from the planet, telling a cheering throng here the United States is ready to lead an international effort to reduce atomic arsenals and the threat they pose.

Speaking only hours after North Korea launched a multistage rocket, drawing new international concern and condemnation, Obama outlined a plan to work toward a goal that he acknowledged remained decades away.

"As the only nuclear power to have used a nuclear weapon, the United States has a moral responsibility to act," Obama told a crowd of more than 20,000 in Prague's historic Hradcany Square. "We cannot succeed in this endeavor alone, but we can lead it."

His speech in Eastern Europe came on a weekend that marked the 20th anniversary of the fall of communism, ending the Cold War that for decades defined American relations with the world.

"The existence of thousands of nuclear weapons is the most dangerous legacy of the Cold War," Obama told the crowd. "Today, the Cold War has disappeared, but thousands of those weapons have not."

On his first tour of Europe as president, Obama then laid out a process to stop the spread of nuclear weapons, ban nuclear testing and secure loose fissile material out of the reach of terrorists. Until weapons and the material to make them are fully secured, Obama said, the United States will maintain its nuclear arsenal to deter adversaries.

Noting the North Korean rocket launch, which many in the West fear was a test of the regime's ability to deliver a nuclear warhead, the president called for that country's leaders and Iran to join in halting the spread of weapons of mass destruction.

"North Korea must know that the path to security and respect will never come through threats and illegal weapons," Obama said.

He added that his administration would engage diplomatically with Iran, another nation pursuing nuclear technology, based on "mutual interests and mutual respect."

North Korea's rocket launch gave added urgency to Obama's long-planned speech. The isolated nation, the president said, faces a decision much like Iran's, as the United States and allies attempt to stem nuclear weapons for both.

"If they want to take an appropriate path to rejoin the international community and break out of their isolation, that's available to them," Obama said of North Korea.

Also in his speech, Obama promised to negotiate a strategic arms-reduction treaty with Russia by the end of the year that would dramatically reduce the number of nuclear warheads. He also called for strengthening of the nuclear Non-Proliferation Treaty, meant to allow access to nuclear power for nonmilitary use and secure nuclear weapons and ingredients from terrorists.

His plan calls for peaceful uses of nuclear power using an international fuel bank that would be available to every nation that renounces nuclear weapons. Obama also said he hoped to host a world summit on nuclear weapons within the year.

The speech received mainly positive reactions from interest groups and arms-control advocates.

"The president is absolutely correct that our long-term goal must be a world free from nuclear weapons," said Sen. John Kerry, D-Mass., chairman of the Senate Foreign Relations Committee.