The Make Poverty History protests in a fortnight give us an incredible opportunity. The leaders of the world's most powerful states, gathering in Scotland, will be faced by the only superpower that should count in the end - global public opinion. Hundreds of thousands of protesters will form a giant white band around the city of Edinburgh demanding justice for the people of Africa. They want an end to the unfair trade rules that suffocate Africa's economy, a doubling of aid, and a cancellation of African debt. They will be reflecting the will of Africa's 900 million people, unrepresented at the global top table.

The Make Poverty History protests in a fortnight give us an incredible opportunity. The leaders of the world's most powerful states, gathering in Scotland, will be faced by the only superpower that should count in the end - global public opinion. Hundreds of thousands of protesters will form a giant white band around the city of Edinburgh demanding justice for the people of Africa. They want an end to the unfair trade rules that suffocate Africa's economy, a doubling of aid, and a cancellation of African debt. They will be reflecting the will of Africa's 900 million people, unrepresented at the global top table.

If these voices are drowned out by aggressive policing and a small minority of violent protesters, this opportunity will be tossed away. It has happened before. Last time there were huge demonstrations outside a G8 meeting was Genoa in 2001. I still remember the body of Carlo Giuliani, a protester gunned down and run over by the Italian police. I remember slipping on a carpet in a youth hostel that had been raided hours before by the Italian police, and realising the crunch beneath my feet came from broken teeth. I remember how badly it all went wrong.

Thanks to the violence, most people remember Genoa - if at all - as a brief whiff of rage and teargas and nothing more. This is a travesty of what Genoa was about, and it will be a travesty if Edinburgh goes the same way.

So what went wrong in Italy that summer, and how can we avoid it happening here? First, the myths must be debunked. The vast majority of protesters who went to Genoa were peaceful and - far from being the "football hooligans" or "anarchist travelling road-show" conjured up by the right-wing press - extremely smart and clued-up.

They were people like Anna Tuit, a Dutchwoman in her mid twenties whom I met. She worked for several years for Médecins Sans Frontières, a humanitarian aid charity that had a strong unofficial presence in Genoa. She explained that she had recently worked in Africa and was protesting that corporate globalisation is denying Aids drugs to dying people.

"I was seeing people who could be treated - whose lives could be extended by 10, 20, 30 years - but we could offer them nothing. The Western pharmaceutical companies do not permit the manufacture of cheap generic Aids drugs in Africa, because they want to protect their copyright and their patents. Our governments put this corporate interest before the human interest. So thousands of people die. I can't just forget the people we turned away."

Or they were groups like - my favourite - the Pink Fairies (dressed as their name demands), who preached the doctrine of "tactical frivolity". They built a "revolutionary spaghetti catapult" to "splatter the leaders with pasta". It didn't work out, but they did succeed in organising a "mass laughing session". When the leaders gathered for their pompous photo-shoot, tens of thousands of protesters simultaneously fell into long, infectious laughing fits, to show - as one protester put it - "how ridiculous, how offensive, how beneath us we find their little power games while so many people are starving". Genoa shook, not with bombs but with laughter.

So how did this sound get drowned out? The main reason is unbelievably aggressive policing. The Italian police weren't just cack-handed; they went out of their way to provoke the protesters, tossing around teargas like it was confetti. I even saw them gas a gaggle of nuns protesting against debt. Hundreds of people determined to protest peacefully saw red and rioted.

Ah, we cluck, but our police aren't so frightfully vulgar, are they? Our police will be the opposite: hands-off and cooling-down - surely? We can't take this for granted. At the recent May Day demonstrations, the Scottish police were notably aggressive, and the Scottish Centre for Human Rights warns they are under huge pressure from the security services of eight countries - and especially the UK national government - to be even tougher.

So what red flags might be waved at the protesters? The police are likely to resort to Section 44 of the 2000 Terrorism Act, as they have across Britain over the past few years. This allows them to stop and search anyone they like, without giving a reason and without any reasonable suspicion, in any area they deem risky.

Many of us would accept these powers if the police used them only, say, to track a Madrid-style train bomber, but they are being routinely used against peaceful protests. In 2003, a group who gathered to protest against the Defence Systems and Equipment International arms fair in east London were detained under Section 44. How are they "terrorists"? Indeed, they were trying to stop the sale of weapons to leaders - like those of Saudi Arabia and Uzbekistan - who terrorise their own people.

The human rights group Liberty predicts that Section 44 will be used in Edinburgh. Memo to the police: tell people they are being stopped under a law designed for mini-Bin Ladens, and you are guaranteed to provoke rage.

That's not all. The police are also trying to block a demo that would pass by the Gleneagles Hotel on 6 July. Why are we giving the security services £10m a PM or president if they can't deal with a simple, peaceful march? In Genoa, this kind of ban - erecting a stupidly large "red zone" in the centre of town where protesters were forbidden - was a boon to the small "black bloc" of violent protesters. Why repeat the mistake?

But the responsibility isn't only on Them; some of it falls to Us too. To the protesters - it is essential that, if the police do play up again, don't let yourselves be provoked. Don't play into the hands of the people who want to dismiss this movement as the ravings of the irrelevant or irreverent. And that goes to the people considering violent protest too. Whatever you think of the ethics of violence - and I'm no pacifist - please realise that in this instance, every violent act will be a gift to the enemies of the African people and of global justice.

Remember how high the stakes are. In the first week of July, the world will only hear one message. It could be the sound of breaking glass and the hiss of teargas. Or it could be the sound of millions of people united behind Nelson Mandela's message to the End Poverty History campaign: "Like slavery and apartheid, poverty is not natural. It is man made, and it can be overcome and eradicated by the actions of human beings. Many of the world's poorest countries remain imprisoned, enslaved and in chains. They are trapped in the prison of poverty. It is time to set them free." It's down to you.

j.hari@independent.co.uk