LONDON (Reuters) - Protesters threw bottles, cans and smoke bombs at riot police in central London on Saturday in a demonstration against the re-election of Britain's Conservative Prime Minister David Cameron.

Scuffles broke out when the 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 officers and another police staff member were injured, Scotland Yard said, in what it described as a spontaneous anti-austerity protest. Two of those hurt were being treated in hospital.

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.

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

(Reporting by Phil Noble and Stefan Wermuth; Writing by Mark Trevelyan; Editing by Jane Baird and Frances Kerry)