It's nearly 10 years ago now, but memories of that late August evening remain as keen as ever. I was reminded again as news reached me that the city of New York is going to pay an $18m settlement for violating people's civil rights when they tried to protest the Republican National Convention in 2004.

I got a little more than I expected when I went for a stroll in New York and walked into Union Square in Lower Manhattan at 6pm on 31 August 2004. In the park, I encountered the full diorama of American resistance, "the Army of Enlightenment, the ragtag Circus of Joy, the Dissident Mob, the Refuse and Resist crowd", I wrote later. Anyone, in short, who couldn't stomach the idea of that foregone conclusion, four more years of George W Bush as president of the United States. They crowded Union Square, handing out fliers, trading notes on the day's arrests, climbing the statue of George Washington and making brave speeches to the people clustered below. It should have been a great demonstration of American democracy. Instead, it turned into a horrific day for many.

I was just back from the sun coast of Florida, where I had actually come face to face with Kathleen Harris, who had made Bush's (arguably fraudulent) first term possible because of the election recount. It was hard not to feel the energy of the people protesting peacefully in the square. It was, it seemed, possible to put up the good fight in America.

And then I made my mistake. I took a flier from a group called the A-31 Marching Band, noticing that they were sprawled out in front of me with their tubas and trombones and drums on the steps facing 14th Street. Marching bands were all the rage back then. They stood up, started playing and headed up Broadway, away from the scene of action, in the opposite direction from Madison Square Garden, which was a good thing since I had agreed to cook dinner at a friend's place.

Big mistake. As soon as we turned down 16th Street, it started. New York police had been massing in great numbers along the east side of the park and, little did we know, on Irving Place as well. Very quickly we were kettled in on both sides of the block, the now familiar but then new flexible orange fencing keeping us from getting out. Easy prey. Although people in Union Square were talking about mass arrests at a "Die-In" near Ground Zero, we were still unaware that the police were arresting everyone they could, without rhyme or reason. Somewhere on the west side in the 20s a whole block of people heading towards Madison Square Garden walked right into waiting police vans and buses. And we were next.

A strange, existential drama unfolded. We couldn't leave even though we weren't charged with anything. We were told to stand still or sit down – that included little old ladies out walking their dogs and restaurant take-out guys. Cops on bicycles appeared and handcuffed us, while menacing riot police glowered from the corners. I can still remember bystanders waiting to see what would happen next, counseling us that if we all behaved, the cops would eventually let us go. It was all a terrific error, a case of mistaken identities. Good luck with that.

What happened next defies imagination. Over a thousand people from all over the city were deposited at a pier, an old bus terminal, on the West Side and hustled in to a large fenced-in area with two benches and toilets for four. The statement by the American Civil Liberties Union (ACLU) this week only captures the facts:

The department then fingerprinted everyone and held hundreds for more than 24 hours at a filthy, toxic pier that had been a bus depot.

Oil and gasoline was spilled everywhere on the floor; a sign announcing the presence of asbestos hung from the ceiling in plain view. We had nowhere to sit, to sleep. If we begged for food, we were given an apple. Our belongings were confiscated; we hadn't been charged with a crime. As the hours passed, more and more arrestees were brought in, we found friends, made new ones. Men and women mixed freely, something rather obviously against the rules. Years ago there was a 30-second video of the scene made on someone's cell phone on YouTube. I don't know if it's still around.

To think that any or all of this just happened is, I know, very American but it ain't so. The NYPD must have rented the pier before the GOP convention and knew precisely what they were going to use it for. Ditto their plans for arresting any mass of people in some way attached to anti-Bush demonstrations. They may even have noticed the asbestos warning and thought: "Too bad for them."

I was in the pier for about a day and then transferred to the Tombs (aka the Manhattan Detention Complex). I crawled out of there three days later (you are only supposed to be held for a day until you go before a judge), clothes, arms and face smeared with oil – but not before gaining a lasting acquaintance with many of the cells and subterranean passageways of that building. They kept moving us around and packed 25 or more into cells meant for five; we were not only fingerprinted again but had our eyeballs photographed. We were lucky to get a sandwich to eat. I befriended a young Frenchman, a student at New York University also out for an evening's stroll, only to encounter an America he had no idea existed.

For that reason and many others, I reserve my doubts about the ACLU's hopeful pronouncement that they "expect that this enormous settlement will help assure that what happened in 2004 will not happen again".