LONDON (AP) _ Demonstrators threw firebombs, cans and bottles at police Saturday during a march against the controversial poll tax. Police said 11 people were injured and 120 arrested.

The clashes began after 2,500 demonstrators left a peaceful anti-tax rally at Brockwell Park in south London and marched to nearby Brixton Prison, where they started pelting police with debris, Scotland Yard said.

Helmeted riot police carrying shields moved in to break up the crowd and scuffled with demonstrators outside the prison and in Brixton's main shopping district.

Six policemen and five civilians were injured, police said.

Busloads of extra police were on duty to prevent a repetition of the anti- tax riot in Trafalgar Square on March 31, one of the worst in central London this century.

That riot left 331 police and 86 civilians injured, caused hundreds of thousands of dollars of damage, and led to 341 arrests.

The protesters marched to Brixton Prison to show support for four prisoners jailed there after the Trafalgar Square riot.

Late Saturday, Home Secretary David Waddington demanded a report from police on how demonstrators were able to run amok in the capital for the second time this year.

The widely unpopular community charge, informally known as the poll tax, is part of Prime Minister Margaret Thatcher's crusade to bury British socialism by hitting at its last bastion - local government.

Since the law went into effect earlier this year, every adult pays a fixed charge, instead of only property owners. Everyone in a district pays the same amount.

The rally Saturday at Brockwell Park was organized by the London Anti-Poll Tax Federation and attended by an estimated 4,000 people.

One of the speakers, opposition Labor Party lawmaker Tony Benn, told the crowd that the campaign against the new tax was getting more and more support.

''There are millions of people who cannot afford to pay the poll tax and they deserve our support,'' he said.

Dave Morris of the Trafalgar Square Defendants Campaign, which organized the prison rally, said police were ''itching'' to attack people who marched there and provoked the violence by hemming in the demonstrators and seizing a megaphone.

Scotland Yard denied the allegation, saying police persistently requested that demonstrators disperse.

Police broke up the protest outside Brixton Prison in about 45 minutes, but smaller groups moved into the Brixton shopping district and continued scuffling with officers and pelting them with bottles, cans, stones and other objects for several hours.

Demonstrators broke shop windows along Electric Avenue, lit small fires and overturned stalls in Brixton market, using them as barricades. Several police vans were damaged and a police motorcycle was damaged by a gasoline bomb.

One policeman was knocked to the ground and carried away by colleagues. Scotland Yard said three of the six injured policemen suffered head injuries. None were seriously hurt.

The poll tax is aimed at bringing home to voters what Mrs. Thatcher sees as the extravagance of the Labor Party's social welfare programs and the inefficiency of the local councils it controls.

Critics say the tax is unfair. Britain's richest man, the Duke of Westminster, pays the same amount under the poll tax as his gardener.