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By Cassandra Vinograd

LONDON — British Prime Minister David Cameron said Friday he will step down after voters moved to withdraw from the European Union.

Official results showed the "Leave" campaign took 51.9 percent of the ballots compared to 48.1 percent for "Remain."

The referendum was Cameron delivering on an election promise — but the premier had forcefully campaigned for Britain to remain in the EU.

The results served as an indictment of Britain's government and immediately sparked speculation about Cameron's political future.

Cameron said early Friday that the British people had spoken — and "their will must be respected."

With markets in freefall after the result, Cameron attempted to reassure anxious investors that the British economy was "fundamentally strong" and that any trade or travel changes would not be immediate.

He said outside of Downing Street that he was "very proud" and "honored" to have led the country for six years — but that Britain now "must prepare for a negotiation" with the European Union, one that "will require strong, determined and committed leadership."

Prime Minister David Cameron and wife Samantha outside Downing Street Friday.STEFAN WERMUTH / Reuters

"I do not think it would be right for me to be the captain that steers our country to its next destination," he told reporters outside his Downing Street office.

A clearly emotional Cameron said he would resign as prime minister by the fall, adding it was "not a decision I've taken lightly."

Former London mayor and leading "Leave" campaigner Boris Johnson has been tapped as a potential successor to Cameron.

He gave a triumphant press conference Friday praising the decision to "take back control" from Brussels, calling it a “glorious” opportunity.

"The British people have spoken up for democracy in Britain and in Europe and I think we can be very proud," he said. "We can find our voice in the world again.

The news hit as Londoners were racing to work, with many glued to their cellphones as the headlines were alerted.

"I'm happy to see him go," said legal assistant Colette, 42.

She told NBC News that she hadn't voted for Cameron — and had cast a ballot in favor of leaving the EU.

"I thought there need to be some big changes," said Colette, who would only give her first name. "It is gonna be a little bit crazy for a while but I think it's going to even out," she added.

Nathan Jacobs, a 24-year-old project manager in London, called the resignation a "really, really big shock."

"I thought honestly he'd push through this stage," he told NBC News.

But Ed Duval, a 40-year-old sales manager, said it was a "brave move" and the "right thing for him to do."

Retiree Jill Bullen was among the millions who cast a vote to "Leave" — though admitted she didn't think her side would win out.

She described Cameron's resignation as "like leaving a sinking ship."

"We need somebody strong to take over and lead us if he's not going to do it," Bullen told NBC News. "We just want our country back, to what England was."

Mo Abbas contributed.