Celebrations have broken out in the United States on news Al Qaeda mastermind Osama bin Laden is dead.

US president Barack Obama announced a small team of American operatives found bin Laden in the Pakistan city of Abbottabad.

After a firefight, they killed him, in what Mr Obama described as the "most significant achievement to date in the effort to defeat Al Qaeda".

US officials said the operation that killed bin Laden lasted less than 40 minutes and also left three other men and a woman dead, including a son of the Al Qaeda chief.

Bin Laden was reportedly identified by the assault force and shot in the head.

The 54-year-old Saudi had been the most wanted man in the world since 2001 when he helped orchestrate the September 11 terrorist attacks on New York and Washington.

"Tonight is a testament to the greatness of our country... we are reminded that America can do whatever we set our minds to," Mr Obama said.

It is the news the United States government has been wanting to announce for years, and the country is celebrating, with huge crowds massing outside the White House in Washington and at Ground Zero in New York City.

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The joyous crowds that descended on the White House spontaneously erupted with chants of "USA, USA".

Some brandishing the US flag, they punched the air and sang The Star Spangled Banner, even before the announcement of bin Laden's death by Mr Obama in a hurriedly-organised address to the nation.

Starting with just a few dozen people, the crowd quickly built to hundreds pressed against the fence outside the White House, in what rapidly became a party atmosphere as Mr Obama confirmed details of the death.

Clayton McCleskey, a journalist with The Dallas Morning News, was outside the White House shortly before Mr Obama began his announcement and says the crowd grew quickly but organically, fuelled by social media.

"There are a couple of universities that are pretty close to the White House and so it appeared that initially it was mostly students," he told ABC News Online.

"It was a very young crowd. There were a few maybe 30- or 40-somethings but a lot of 20-year-olds.

"Everybody had their phones out. Everybody was on Facebook. Everybody was texting. I heard a lot of people on their phones saying 'hey, come meet me'."

McCleskey says his immediate reaction upon seeing the crowd - which he estimated at 2,000 - was "oh gosh, this is horrible. This is just like a drunken soccer party".

"There were people who had taken out bottles of vodka and were drinking in the streets and were running around with American flags," he said.

But McCleskey says while there was definitely a euphoric mood, other quieter and less visible members of the crowd were taking the historic occasion more seriously.

"You had the sense that this was the moment of victory in the 'war on terror' that so many Americans had been waiting for. That was a very joyous, jubilant feeling," he said.

"I saw some people with tears in their eyes. Some people were singing God Bless America.

"As I went around and interviewed people in the crowd, I was really surprised by how many people said 'look, this is a jubilant occassion, but we have to remember the sacrifices that so many people have made in order for this to happen'."

Kenneth Specht, a New York firefighter on 9/11, paid tribute to the victims of the attacks in New York and Washington

"I'm proud to be an American tonight," he told CNN.

"Tonight they are first and foremost in our minds."

"It's a long time coming," hotel clerk Becky Grant, 26, at the Fairfield Inn in Ann Arbor, Michigan, said, before asking a passing guest: "Did you hear the news? Bin Laden's been killed.

Linh Do, a Melbourne resident travelling in the United States, was on a bus arriving in Washington DC when she heard Mr Obama was about to make a national security announcement.

Rumours were circulating that bin Laden was dead and Ms Do, sensing a historic opportunity, made her way straight to the White House, joining a crowd of about 100 mostly young people which quickly multiplied.

She says although the crowd was embracing of everyone there, she felt like an outsider.

"The feeling was definitely one of American patriotism. It was a bit like the fourth of July but pumped up on steroids," she told ABC News Online.

"People had made quick banners or messages with things like 'Obama's killed Osama', 'We never forget' and 'We never lose'."

Ms Do says among the sea of American flags and other patriotic paraphernalia she noticed Obama 2008 election campaign T-shirts and even some Obama 2012 material.

She says while some parts of the crowd were celebrating with abandon - she could even smelt marijuana at one point - others were more circumspect about bin Laden's death.

"From a distance, and even if you are there, it does seem like a bit of a celebration... not everyone is viewing it like that," she said.

"There are obviously still sane and sober people there."

New York celebrates

In New York, where some 3,000 people died when Al Qaeda hijackers crashed two commercial airliners into the the Twin Towers, tourists and New Yorkers descended on Times Square as well as at Ground Zero.

"It's a miracle," said New Yorker Monica King, 22.

"The attacks changed New York and now 10 years later we had our last word. Now we want to celebrate."

Gary Talafuse, visiting from Texas, said Americans "feel a lot of national pride".

"This may not change anything in Al Qaeda tactics, but after billions of dollars invested, this is a big loss for Al Qaeda, and that brings some degree of reward to our efforts," the 32-year-old said.

Even the New York Police Department, usually low key, parked a car in the middle of Times Square and started talking to people, laughing and posing for photos.

In Los Angeles, where the news was confirmed shortly before 9:00pm (local time), a roar of applause could be heard from bars with TV screens on the Santa Monica Boulevard in West Hollywood.

"After so much time trying to get bin Laden this is a big surprise and amazing news," said Mick Pleasant, 26, at the Cabo Cantina bar.

"Terrorism is down, and Obama's speech was great."

"I'm really happy that this guy is dead because he killed so many people around the world. I'm glad that our nation is save because of this," said Cesar Guellory, 25.

Carol Morrison, 35, said she had voted for Mr Obama.

"He's showing us how strong he can be... Today we can say that we are safe. I'm proud of United States," she said.

Websites including Twitter and Facebook were also deluged with reaction to the Al Qaeda leader's death.

- ABC/AFP/Reuters