WASHINGTON - President Bush took the nation to the brink of war last night, warning that unless Saddam Hussein and his sons went into exile within 48 hours, the United States would invade Iraq and topple his regime.

Speaking from the White House, the president sought to prepare Americans for a war with unknown consequences that will begin, he said, "at a time of our choosing."

He said he was abandoning all diplomatic efforts at the United Nations, which refused to provide the authorization for military action that Bush had sought.

"All the decades of deceit and cruelty have now reached an end," the president said.

"The tyrant will soon be gone," he said.

Defiant to the end, Hussein gave no sign of heeding Bush's demands, Associated Press reported early today. Hussein warned that American forces will find an Iraqi fighter ready to die for his country "behind every rock, tree and wall."

But he made a last-minute bid to avert war, admitting that Iraq had once possessed weapons of mass destruction to defend itself from Iran and Israel - but insisting that it no longer has them, AP reported.

"We are not weapons collectors," the official Iraqi News Agency quoted Hussein as telling Tunisian Foreign Minister Habib Ben Yahia, who was visiting Baghdad in a last-minute quest to avert war.

"When Saddam Hussein says he has no weapons of mass destruction, he means what he says," Saddam said.

Bush urged aid workers, Western journalists and others to leave Iraq before an attack begins. Several countries have closed their embassies in Baghdad, and United Nations Secretary-General Kofi Annan ordered international weapons inspectors and relief workers out of the country.

Israel, which is bracing for a possible counterattack by Iraq, urged residents to create protective spaces in case of missile attacks.

The president, speaking somberly for 15 minutes from an East Wing hallway, said the nation had no choice but to act now to disarm Hussein of his weapons of mass destruction, which he said terrorists could use against Americans.

"The United Nations Security Council has not lived up to its responsibilities," Bush said, "so we will rise to ours."

The president sought to answer doubts raised by other world leaders and by some Americans about why Iraq must be confronted immediately, given the risk of retaliatory terrorist attacks and questions about whether the United States has legal standing to invade a nation that has not provoked war in any obvious way.

Bush said he understood that war could put Americans at greater risk of terrorism. And he ordered the government to raise the nation's terrorist threat level to orange, or "high." The higher level will mean heightened patrols at seaports, airports and nuclear power plants.

"Should enemies strike our country, they would be attempting to shift our attention with panic and weaken our morale with fear," Bush said.

But, he said, "we will not be intimidated by thugs and killers."

Bush argued that U.N. resolutions dating to 1990, when the international body authorized his father to lead the United States into war against Iraq, remain in effect and provide legal backing to disarm Iraq.

"The United States of America has the sovereign authority to use force in assuring its own national security," he added. "That duty falls to me, as commander in chief, by the oath I have sworn, by the oath I will keep."