My relief was premature. As a squad of riot police filed past us, one of them jabbed me in the stomach with his truncheon and, while I was bent double, another said: "Let him eat potatoes." From somewhere I recalled that the phrase "mangiare patate" was Italian police slang for beating the living daylights out of someone.

It was not quite what I had in mind when I joined a convoy of two coaches organised by Globalise Resistance - an anti-capitalist group - that set off from London last Thursday.

Nearly half the protesters on my bus belonged to the Socialist Workers party (SWP) and the rest to various unions. They wore T-shirts proclaiming: "Team Marxism" and baseball caps emblazoned with "Unison" and the T&G logo.

Many were in their teens and twenties, sporting beards, dreadlocks, combat trousers and dirty trainers. Some, such as Colm Bruce, seemed to have spent a lifetime working for left-wing causes.

He was leading a group of 10 Irish SWP members and is writing a book on shootings by the British Army in Northern Ireland.

"I feel like a Spitfire pilot about to go into battle," said Max, a T-shirted student with a goatee beard and cropped hair.

Everyone was in high spirits. Dave, a youth worker from Nottingham with a SWP scarf covering his head, pulled on a latex Tony Blair mask and waved at passing traffic.

Many passengers were veterans of political demonstrations. Dave Ramsden, a tough-looking hard-leftist in his forties from Bradford, boasted about his exploits at the Prague anti-capitalist protest last year. "We got the opera closed down," he said. "We surrounded it so no one could get in. Then we had a beer."

Bruce held uncompromising views on police casualities. "They are racist, sexist, violent scumbag bastards," he said. "I know coppers who've been injured and they deserve everything they've got."

By contrast John, from Liverpool, who works for an anti-capitalist group called Humans Not Profit, emphasised the need to avoid violence. "The thing is to be low-key," he said. "Leave the heavy stuff to people like Ya Basta [the Italian militant anti-capitalist group] or to people who have gone over and trained for it."

John had visited Ya Basta in Milan recently to talk about strategies for occupying disused properties and also about tactics for Genoa. Other protesters said "rebel groups" from Britain had travelled to Italy to train for the Genoa protests.

As to exactly why they were heading for Italy, many were vague. They were apparently impelled by a desire simply to vent their dislike of big business, the power wielded by the most economically successful nations and the Blair government's lack of interest in left-wing causes.

John Worthington, a thick-set man from Birmingham with tattoos on his arms, said: "I'm going for an adventure." He added: "It's about everything. But if it all goes off, I'm going to run. But if the police hit me then I'm going to hit back."

At Dover our baggage was searched and our pockets checked. The first drama was to take place at Calais.

French customs officers searched our bags and kept us waiting for two hours before one passenger was deported back to Britain when a Russian gas mask was found in his baggage. We loudly sang The Internationale.

At the Italian customs post at Bergamo, there were random searches and two passengers were refused entry.

As we approached Genoa, Bruce filled us in on direct-action tactics. "The aim of today's demo is to break into the red zone," he said, referring to the inner area of the city where the G8 leaders were sealed off. "The only way of preventing the summit going ahead is militant, non-violent direct action. You link arms, do things collectively and march against the police line, push against it if you feel that's appropriate. We sit down, link arms and don't let anyone get arrested."

At Genoa we made it to a "peace area" in the hills above the harbour, where hundreds of British protesters were milling about in the sunshine.

Then the news spread: there was trouble in the city centre - riots, some said. I ran down the steep alleys and streets to the Brignole railway station, planning to take a quick look and then return. I found the aftermath of a riot, marked by pungent tear gas, smashed phone boxes, burnt-out cars and trucks lining a wide street.

Seeing activity 200 yards away, I put on my cyclist's helmet and walked over to an arena flanked by hundreds of police in helmets, masks, body armour and shields, with press photographers roaming about. A huge roar came from the other end of the street. I ran towards the noise, pulling on my swimming goggles and cyclist's anti-smog mask. I could make out hundreds of brightly coloured figures descending a steep street. A blizzard of stones and rocks flew towards a group of police.

These were the Italian anarchists - teenagers and twentysomethings, many wearing foam body armour, helmets, combat trousers or ice-hockey masks.

Prepared for battle: a helmeted protester, engulfed by tear gas, hurls stones at police. Photograph: Nick Cornish

A pattern developed over the next half an hour. Protesters surged forward, throwing rocks and smashing up phone boxes and dust carts, before police could fire tear gas, forcing the protesters, eyes and lungs burning, back into other streets. There they would douse their faces with water, lemon or vinegar. Tear gas doesn't just restrict your breathing but sticks to your skin, irritating it like a dose of sunburn.

Then a carabinieri van hurtled into the street, lights flashing and horn blaring. Immediately, half a dozen heavily armoured protesters set upon it, smashing windows with rocks and sticks. After two minutes a masked protester was standing on top of it waving a hammer and sickle flag; within five minutes it was on fire.

Spurred on by this, protesters began to run forward and throw more rocks at the 100-strong police line, some pushing dustbins forward to use as shelter. Others appeared with huge plastic shields, 5ft x 10ft and backed with scaffold frames. They dashed forward behind these every few seconds to hurl more rocks at the police.

What had seemed like only a couple of hundred protesters had swollen within half an hour to several thousand. They swarmed around the streets and stations, standing on cars to get a better shot with their rocks.

Teams in makeshift medical clothing bandaged those injured by the flying masonry. No sooner had the tear gas dispersed than protesters a little further down the street hurled the canisters back at the police. The officers, in turn, inched forward behind their shields, picked them up and threw them back again.

In the midst of this mêlée, the death of a young Italian anarchist was witnessed by Nick Cornish, a 35-year-old photographer who captured it on camera yards away.

All day, he had pursued groups of protesters careering through side streets, some brandishing weapons and with their faces masked. "I had been running from one square to another. There was a lot of confusion," he said.

Cornish watched two carabinieri vehicles manoeuvring on a main steet that had been blocked by overturned dustbins. As a handful of protesters closed in on one vehicle, he aimed his camera from some steps - the only vantage point.

"I saw these two police vans and one of them was in trouble. I heard a couple of shots but I did not realise what they were at the time. I noticed someone under the vehicle and then the car drove over him and drove away. It was obvious he was dead."

This was about the time that I ran into trouble, receiving beatings that turned my arms and back a technicolour hue.

It was not until I was marched to the police station that the mood calmed. After 10 minutes they took the handcuffs off and it became clear how frightened they were, too. I was taken to wash and a policeman poured antiseptic on a deep cut in my wrist caused by the handcuffs.

The clock ticked by. Every time a policeman I had not seen before entered the office, he looked at me as if he wanted to bash a chair over my head.

Two-and-a-half hours after I was hit on the back of the head, the chief undercover officer said I could go. Before I left, another prisoner, a long-haired, skinny teenager who sat on the floor, his hands bound behind him, mouthed silently: "Help me. Get a lawyer." I nodded.

On the way out, I recognised one of the men who had beaten me up. He told me in a friendly fashion to hold my press card up at all times and look all around me as I walked. As advice it was rather tardy.