Declaring “justice has been done,” President Obama announced late Sunday that Usama bin Laden was killed by U.S. forces in Pakistan, marking the end of the worldwide manhunt that began nearly a decade ago on Sept. 11, 2001.

The president made the stunning announcement within hours of informing congressional leaders. He said bin Laden was killed Sunday, the culmination of years of intelligence gathering. The news drew a large crowd to the front of the White House, as well as in Times Square, as people chanted “U-S-A. U-S-A.”

Obama, in his address to the nation shortly before midnight, thanked the Americans who have toiled in pursuit of bin Laden and applauded those who carried out the successful mission in Pakistan. Describing that mission only briefly, he said its result “is a testament to the greatness of our country.”

“For over two decades, bin Laden has been Al Qaeda’s leader and symbol,” Obama said. “The death of bin Laden marks the most significant achievement to date in our nation’s effort to defeat Al Qaeda.”

The president traced the death of bin Laden to a tip received last August. He said he was briefed at the time on the “possible lead,” and that after months of intelligence work it was determined bin Laden was hiding in a compound “deep” inside Pakistan. Obama said, after determining the intelligence was sound, he authorized the operation to bring him to justice last week.

More On This... Usama Bin Laden

He said a “small team” of Americans went after bin Laden in Abbottabad on Sunday. “After a firefight, they killed Usama bin Laden and took custody of his body,” the president said.

Senior administration officials, in a briefing with reporters, afterward said the administration had determined by February that they would pursue the compound "custom built to hide someone of significance" in Pakistan. This decision led to a series of national security meetings starting in March to develop a course of action. Obama gave the final order to pursue the operation on April 29, officials said.

The house was 100 yards from the gate of the Kakul Military Academy, an army-run institution where top officers train. A Pakistan intelligence official said the property where bin Laden was staying was 3,000 square feet.

At 3:30 p.m. EST, a 40-man Navy Seals squadron raided a compound in Abbottabad, Pakistan, killing the Al Qaeda leader with a bullet to the head.

Four Chinook and Blackhawk helicopters dropped 24 men on the compound. One helicopter suffered a "hard landing" inside the compound after experiencing a mechanical failure and had to be destroyed on the site, according to one defense official.

There was a large shootout. The residents at the compound resisted. The total raid took 40 minutes.

No Americans were killed in the mission Sunday. Officials said three adult men other than bin Laden were killed – one was believed to be bin Laden’s son, the others couriers. One woman was killed when she was used as a human shield and two other women were also injured, the officials said.

Abbottabad resident Mohammad Haroon Rasheed said the raid happened about 1:15 a.m. local time.

"I heard a thundering sound, followed by heavy firing. Then firing suddenly stopped. Then more thundering, then a big blast," he said. "In the morning when we went out to see what happened, some helicopter wreckage was lying in an open field."

"Intelligence analysts concluded that this compound was custom-built to hide someone of significance," he said.

In the wake of bin Laden’s death, authorities around the world are being urged to take security precautions. One source said officials are concerned bin Laden’s death could incite violence or terrorist acts against U.S. personnel overseas.

The State Department issued a travel alert for U.S. citizens abroad overnight, citing “the enhanced potential for anti-American violence given recent counter-terrorism activity in Pakistan.”

Obama said Americans must continue to be “vigilant.” But he said the death of the architect of the deadliest terror attack on U.S. soil should be welcomed around the world.

“Bin Laden was not a Muslim leader. He was a mass murderer of Muslims,” Obama said. “So his demise should be welcomed by all who believe in peace and human dignity.”

Sources said the vice president informed congressional leaders late Sunday night that the world’s most wanted man had been killed.

Officials said bin Laden’s body, which was in U.S. custody, was given a sea burial.

The announcement comes nearly a decade after the 2001 terror attacks that triggered the Afghanistan war and started a tireless hunt for the terrorist mastermind and Al Qaeda leader.

In recent years, that hunt had increasingly led U.S. intelligence across the border and into Pakistan, where Al Qaeda is thought to be concentrated.

Fox News' Jennifer Griffin, Justin Fishel and the Associated Press contributed to this report.