The protests erupted against the Conservative Party, after prime minister David Cameron was re-elected.

A protest has erupted in central London against the re-election of Britain's Conservative prime minister David Cameron, with demonstrators throwing bottles, cans and smoke bombs at riot police.

Scuffles broke out when the anti-austerity demonstrators, blaring hooters, banging pots and chanting obscenities, confronted lines of police outside the gate leading to the prime minister's Downing Street residence. At one point a bicycle was hurled at police.

Police arrested 17 people, and four police officers and one member of police staff were injured during the protest, a Scotland Yard spokesman said.

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A Reuters photographer estimated that a couple of hundred people took part in the protest, including a group of about 25 black-clad youths with sunglasses and face masks.

He said police briefly closed the central Whitehall Avenue to traffic but later reopened it and surrounded the last remaining group of several dozen protesters.

Anti-Tory graffiti was also daubed on a war memorial honouring the women of WWII in what the Royal British Legion called a "senseless act".

Green Party activist Elliot Corner said the UK needed a proportional representational system.

"We are here because we've seen bankers get off scot-free while the working people are the only people who've been punished under the rabid schemes of austerity," he said.

Mr Cameron, whose government has enacted tough spending cuts to bring down the budget deficit and promised more to come, won a second five-year term in Thursday's election with an outright majority in parliament.

His pitch to voters was that he had rescued Britain from economic crisis to deliver the fastest growth among major economies.

He had warned that Labour leader Ed Miliband would cripple the UK by giving Scottish nationalists the keys to England's treasure.

But the Scottish National Party (SNP) pledged to end austerity measures after making massive gains in the election, rising from six to 56 out of the 59 parliament seats for Scotland.

"The people of Scotland on Thursday voted for an SNP manifesto, which had ending austerity as its number one priority," SNP leader Nicola Sturgeon said.

"That is the priority that these men and women will now take to the very heart of the Westminster agenda."

Mr Cameron's Conservative Party can rule in its own right with a majority of 12 in a 650-seat parliament, although it plans to meet with the emboldened Scottish nationalists.

Reuters/AFP