GLASGOW, Scotland -- The last time Britain held a national election, Mhairi Black wasn't old enough to vote. On Friday, the 20-year-old university student was elected to Parliament as part of a Scottish nationalist wave that has transformed British politics and raised questions about the country's future.

Black defeated Labour bigwig Douglas Alexander, an upset that symbolized the scale of the Scottish National Party's triumph. Seven months after losing a bid for independence from England, the party took 56 of Scotland's 59 seats, ending decades of Labour dominance.

"The people of Scotland are speaking, and it is time for their voice to be heard at Westminster," said Black, the youngest British lawmaker since 13-year-old aristocrat Christopher Monck was elected in 1667.

The SNP is now the third-largest group in Parliament after the Conservatives and Labour, an ironic position for a party that wants to break off from the United Kingdom.

Scots voted 55 per cent to 45 per cent to remain in the U.K. with England, Wales and Northern Ireland after a concerted effort by all of the country's major political parties to persuade them to stay.

But the aftermath of the referendum and the failure to immediately deliver on promises made during the campaign infuriated the Scots, who felt betrayed by a political class that they saw as having long ignored their desires and aspirations.

The SNP victory made the prospect of a new independence referendum more likely, though leader Nicola Sturgeon has stressed that the party did not campaign on a new bid for independence.

Nicola McEwen, a politics professor at the University of Edinburgh, said the election outcome would be "a significant test for the future of the U.K."

"It depends very much on how much influence the SNP can exert within Westminster, and how willing the Conservative government is to respond in a way deemed acceptable to the people of Scotland," she said.

Although the pro-independence side lost the referendum, their campaign energized young people like Black who want an end to austerity programs to curtail government spending imposed by Conservatives and remove Britain's nuclear-armed submarine fleet from Scottish waters.

Five years ago, the SNP won only six seats and just 20 per cent of the vote. Now, with 56 seats and more than 50 per cent, the party believes it is in a position to push Scottish issues to the forefront of British politics.

"For years they have said that only Labour can stand up to the Tories, but if they can't convince enough English voters to back them it doesn't matter what we do," said Alan Pollock, 49, an SNP volunteer from the Glasgow area. "This time we have called their bluff and voted for a party that will stick up for Scotland."

Jim Murphy, leader of the Labour Party in Scotland, lost the seat he had held for 20 years to Kirsten Oswald, a human resources professional who only joined the SNP 11 months ago.

Sturgeon has been a star of this election even though she wasn't running for office. The party has seen membership rise from 25,000 to more than 105,000 in six months.

"The tectonic plates of Scottish politics have clearly shifted and what we have seen is a historic shift in Scottish political opinion," Sturgeon said.

Among the SNP politicians aiming to shake up the London-based political establishment is former First Minister Alex Salmond. He resigned last September after losing the independence referendum but is now making a political comeback after winning a seat in the British Parliament.

"It's a very positive vote from an electorate that has been energized and electrified by the referendum process," Salmond said of the SNP performance. "We are not the same country as we were a year ago."

Associated Press Writer Sylvia Hui contributed to this report.