Suddenly they were there. The Albert pub is a Liverpool football club landmark. It sits on the boundary of Anfield, in front of the Kop. It's a place to gather before games, to sing songs, meet friends and try to fight your way to the bar. Last night, at about ten minutes past seven, 35 minutes before the Champions League semi-final between Liverpool and AS Roma kicked off, I was among a small group of fans milling around in front of the Albert.

We were outside the Albert and then, out of nowhere, so were 50 or so Roma fans. Ultras – the hardcore – in balaclavas and face scarves, swinging belts and bats and ropes, singing a song about Liverpool, hissing it at us as they appeared in front of us. Ratty guys in in pre-distressed jeans and big white trainers. One of them was carrying a hammer. One of them had what looked like a hangman's noose in his hand.

It was a routine face-off, and then it wasn't. There was a flurry, a blur of activity, a belt was swung and then there he was, a middle-aged man laid out on the floor, a few yards from me, unconscious. A minute or so before I'd noticed him, noted that he was about the nearest one of us to the ultras. From where I was standing, he could not have looked more harmless. A middle-aged man minding his own business in front of the pub. A fan standing outside the ground before the game, taking it all in. And then he was on the ground.

The ultras charged and we, the Liverpool fans, scattered. Then, in a moment of collective realisation, we came back on ourselves, understanding that surely there were more of us than there were of them.

Fighting had broken out all around the Albert, but the man still lay unmoving in the midst of it all. He must have remained like that for at least five minutes. With the ultras intent on attack, it was hard to know what to do. Everywhere was confusion.

Paramedics showed up and a path was cleared to him. The police had been nowhere to be seen, and when they finally showed up, a teenage boy next to me – a Liverpool fan – went for them. Furious and upset, he wanted to know where they'd been, he wanted to know why they hadn't been doing their job.

It had seemed as though the Roma contingent had come out of nowhere, but in fact they had come down Venmore Street and out onto Walton Breck Road, which runs past the Albert and the Kop. I got in between the police and the teenage boy. "Tell him to calm down," they told me.

(Photo by Oscar Rickett)

The middle aged man was, by this point, being attended to by paramedics. His name is Sean Cox, a 53-year-old from County Meath in Ireland, a man described as a long-standing and popular member of the St Peter's GAA football club in his hometown of Dunboyne. Two men from Rome, aged 25 and 26, have been arrested on suspicion of attempted murder. Sean Cox is in the Walton Neurological Centre. Merseyside Police confirmed that he suffered a head injury and that his "condition is described as critical".

Inside the ground, only a small minority of fans knew what had happened. It gave the game an edge for some of us, a feeling that justice needed to be served. I thought of the range of feelings the ambush had provoked: shame at having retreated, fear of what might happen, and that confusing, dreadful excitement that can come from violence. I thought of Sean Cox, though I didn't know his name at the time.

After the game – the incredible game, a game that had us on the Kop looking at each other in amazement – I walked back to where I was staying, next to Toxteth cemetery. A-roads and Victorian buildings, seagulls and the moon coming out from behind the clouds, fans singing Allez allez allez. I got back to my room and found out about Sean Cox, found out he was in a critical condition.

This morning, BBC Radio Four reported on fights "between" Roma and Liverpool fans. That's not what it looked like to me. What it looked like was an ambush on fans who didn't want to fight by fans who did want to fight. AS Roma have condemned the actions of a "minority" of their fans, though this is by no means the first time this has happened.