Away from the beaches of all-inclusive resorts, Jamaica has one of the world's highest murder rates. In 2005 - a record year - 1,674 Jamaicans were killed, the majority young men gunned down in gangland shootings. The World Bank has concluded that, if the country reduced its murder rate by a third, its economic growth would double.

But gangs are not the only killers. Jamaica has one of the highest per capita police killing rates in the world, comparable to Colombia. According to the government's own figures, the police killed 277 people in 2006. By July this year the number had reached 155. Over the past few weeks there has been a police killing nearly every day.

Driving across Kingston is dangerous. Swaths of the city are warring ghettos, known as garrisons. A million people, about a third of the island's population, live here. Streets are cut by roadblocks manned by gunmen who demand money for safe passage. Communities are isolated behind front lines marked by burnt-out cars and smouldering oil drums.

On the dusty streets of Tower Hill, one of Kingston's most violent slums, a group of policemen clutch assault rifles. Thirty minutes before, their colleagues had shot a man and an angry crowd had gathered. Witnesses tell us how a police patrol stopped an armed man who surrendered, putting up his hands. They claimed he was surrounded and then thrown to the ground. They heard the man plead for his life before police shot him at close range. The officer in charge said: 'Police only shoot in self-defence.'

But Andrew Holness, the opposition Jamaican Labour Party (JLP) MP who represents Tower Hill, said it was the ninth questionable killing in those streets since last year. There was an attitude that anyone found carrying a gun 'deserves to be executed'. Innocent people, he believed, were being killed for no reason, and the accused dispatched without recourse to the due process of law.

Peter Phillips, Jamaica's Minister of National Security for the past five years, accepts there is corruption. In July he announced a review of police practices including human rights abuses. An expansive figure sitting behind an expansive ministerial desk, he does not deny the findings of an opposition party report which accused his officers of 'selling ammunition, planting evidence, removing evidence from crime scenes, perjury, extortion and contract killing'.

'There are circumstances of police killing that are questionable and that we have investigated,' he confirms. In the past eight years, with almost 2,000 police killings, only one police officer has been convicted of murder. The Court of Appeals president, Justice Seymour Panton, has called for an end to the 'appallingly high rate of extra-judicial killings'. In his opinion, police testimony in such cases is 'no longer generally credible'.

Getting into Kingston's garrisons is not easy. David Chang, a convicted murderer turned evangelical preacher, volunteered his services as a guide into Majesty Gardens, where arriving uninvited can be fatal. This garrison has the island's highest murder rate, and it is also in the Prime Minister's constituency. There are no signs of tourist dollars here, nor the benefits of the UK write-off of £39m of Jamaica's debt. Children play in open sewers and the smell of blazing rubbish stings the throat. All water is fetched by bucket from standpipes. What few men remain visible are mainly gang members, or drunks. Blinking against the ash and dust, Pastor Chang spread his arms and proclaimed the social panacea of the Gospels. There were few takers for his message. On the site of his Covenant Community Church he admitted he was kept 'very busy with funerals'.

Omar Davies, Jamaica's Minister of Finance, has in the past been accused of having gangland links, allegations he denies. We caught up with him as he reviewed a gully construction project in Rose Town. A once affluent area of Kingston, much of Davies's constituency has since been dismembered by violence. The gang leaders, he agrees, are used by politicians 'to enforce order', because the police are ineffectual.

Listening to Davies was one of his young constituents, 12-year-old Kareem Martin. Once the minister had moved on, he told us how he'd seen a 15-year-old girl shot dead on this street. Asked how many other people he knew had been murdered, he shook his head: 'I can't count, sir, I cannot count.'

Last week Bishop Herro Blair, the man responsible for monitoring the electoral process in Jamaica, claimed political parties appeared to have ended their links with criminal gangs. We met half a dozen young men with a different story. They said what had started as an internecine struggle within the One Order gang had become 'a little bit political'.

The youth wing of the government party approached the One Order splinter group with a deal: provide the PNP with a foothold in a Labourite area, and in turn receive weapons, ammunition and gunmen. The deal was sealed: would they now provide votes for the government? 'Them expect that,' rasped their leader, 'them expect that'.

· Director James Brabazon travelled to Jamaica with reporter Evan Williams. Their film 'Guns, Votes and Money' opens Channel 4's new series of 'Unreported World' on 14 September at 7.30pm.