Soldiers! Your labours, your privations, your sufferings and your valour will not be forgotten by a grateful country." The inscription on the plinth below a statue of Sir Henry Havelock, Major General of the British Army during the 1857 campaign in India, is as potent as it is appropriate.

The monument stands guard at the south-eastern corner of Trafalgar Square, casting a steely eye over the junction between The Strand and Northumberland Avenue. It's an unusually tranquil pocket of central London today, as tourists roam around the rare open space, and the fountains drown out the rumbling of the London traffic. And yet it was here, 20 years ago on the final day of this month, that flames sent smoke billowing into the blue skies above the capital as protests against the Poll Tax escalated into some of worst rioting post-war Britain has seen.

Tube stations were closed and much of the capital had to be cordened off as cars and police vans were set alight, shops looted and police and firefighters were pelted. Perhaps the most enduring image from the day is the shocking sight of a woman being trampled by a charging police horse. According to David Maynell, the then deputy assistant commissioner of the Metropolitan Police, trouble sprang from only 3,000 or so protesters out of a total estimated at up to 200,000, but the fall-out was far-reaching: £400,000 worth of damage, 400 arrests, 113 injured and, arguably, the downfall of Margaret Thatcher, who resigned leight months later.

The Poll Tax was a flagship policy of the Thatcher government. It had been implemented in Scotland a year earlier and its deployment south of the border was looming, slated for the start of the new tax year the following month. The tax would see the abolition of rates based on the value of a property, replaced by a fixed charge per adult resident and set by each local government. In practice, critics pointed out, that meant a millionaire living alone in a mansion would pay less than the average family, but Thatcher, defending the tax earlier that month in the Commons, argued that it was "a very much fairer system than domestic rates which preceded it", where, "a single person in one house would pay the same rates as four or five people in the next door house."

Few agreed with her. Particularly the All Britain Anti-Poll Tax Federation (the Fed), a national body of anti-Poll Tax unions set up by Militant Tendency, forerunners of today's Socialist Party, who organised a mass protest march to take to the streets of London and Glasgow simultaneously.

Saturday, 31 March 1990 was an unusually warm spring day, and by 11am a crowd had begun to gather for the initial rally in Kennington Park, south London, ahead of a march to Trafalgar Square. Steve Glennon,the Fed's chief steward, says it felt different to the average demonstration. "People who had never been on a demo before in their lives came out. That was how far-reaching an issue it was. There was a sea of people and a real carnival atmosphere – people with kids had come out with the little ones in pushchairs."

Drummers, bands and jugglers lent a colourful air, but didn't dilute the focus. "I remember an electric atmosphere," says Chris Bambery, then 34 and protesting with the Socialist Workers' Party. "It was really positive and everyone was keen to make a stand," including, he recalls, the miners. "They saw it as another chance to have a go after everythingThatcher had already put them through. I remember a real feeling that things are changing here – we had been taking it on the chin repeatedly but enough was enough."

Thatcher's popularity had been sliding. According to a March 1990 Mori poll for the Sunday Times, support for her among the electorate stood at just 15 per cent.

While the overall atmosphere was positive, problems began to emerge. At the last minute, police changed the agreed drop-off points for more than 800 coaches, sending stewards to the wrong places. At 1.30pm, the crowd set off on the two-and-a-half-mile journey to Trafalgar Square, where speakers including George Galloway and Tony Benn were to address them. The 70,000-capacity Square filled up in no time; police had closed off much of the immediate area, at the organisers' request, creating space for 190,000 people in total. Three weeks earlier, organisers had requested that the march be diverted to Hyde Park to accommodate the crowd. The request was denied.

Jane Spencer, from north London, was 22 at the time, and went along with several friends. "It was just such an evil tax," she says. "It played entirely into the hands of the wealthy and a lot of poorer people couldn't afford to pay it." (As part of the protest, an estimated 17 million people in England also refused to pay when the tax did take effect in April. When the tax was abolished under John Major's leadership of the Conservative party, in 1993, councils were £2bn in arrears as a result.)

Ms Spencer adds: "There was nothing like being around such a strong group; we felt protected and untouchable – we could really change something. It felt like we were going to overthrow the State."

At around 4pm, Steve Glennon got a report from a stewards at Downing Street that a sit-down protest in nearby Whitehall had become "a bit of a ruck" between police and protesters. This was to be the turning-point.

As protesters refused to be moved, the police began making arrests – only to be pelted with missiles. The ripple of violence spread quickly to Charing Cross Road, Pall Mall, Regent Street and Covent Garden. "Next thing I knew," says Chris Bambery, who was in the Square, "there were police charging up Whitehall. At that point we didn't know what was happening at Downing Street. The crowd just responded and ran at them, and the riot cops ran away. That really stands out for me, the moment I saw the cops turn and run. That's when I knew this was different to any other demonstration I had been to."

Carmela Ozzi, then 29, from west London, was protesting with her then-partner and his brother and remembers vividly the feeling of unease that spread through the crowd. "The tensions had started before we got to Trafalgar Square; then things got really bad really quickly once we were there." She was caught in the thick of the action when the police returned to charge the baying crowd. "It was very scary. We were near a van that was driven into the crowd, and with the horses we were being pushed back and back into a corner. I remember thinking, 'Somebody is going to get hurt here today'. I felt we were penned in like caged animals." The trio got caught up in a charge; a scuffle ensued and she says she and her partner were arrested, trying to protect his brother. "That day was a defining moment for me, and dreadfully painful; I knew I would never look at the police the same again."

Tommy Sheridan, who was chairman of the Fed, was preparing to address the crowd in Trafalgar Square when the violence erupted. "I was standing next to George Galloway and his daughter, listening to Tony Benn speak, when it all kicked off. George was very worried for his daughter's safety because there were bits of wood and debris being hurled at the police."

Meanwhile, protesters had set fire to builders' Portacabins at a building site on the corner of The Strand and Northumberland Avenue, lacing the air with the acrid smell of burning rubber.

Meanwhile, according to Steve Glennon, a police commander was standing on the empty plinth in front of the National Gallery co-ordinating charges. Glennon pleaded with him to call off his men in an attempt to quell the growing panic.

"We had agreed with the police that if Whitehall became blocked we would go through north Embankment and Northumberland Avenue, which we did," explains Glennon. "But then there was a phalanx of riot police right across the top of Northumberland Avenue stopping people getting out and causing a pressure point." Unable to escape, he says, fleeing protestors began to panic. "You had this great mix of people, right down to small kids," adds Glennon, "and yet they were all getting repeatedly charged by police horses."

Ken Smith, then the press officer for Socialist Party newspaper Militant, arrived at the Square mid-riot. "I heard it before seeing it," he says. "At first it didn't register that the noise was connected with the demonstration. I remember a lot of sirens. It was bewildering how something so positive had ended up in confrontation. The approach by the police had been a big contributing factor."

The Metropolitan Police's David Meynell, thowever, told the BBC at the time that a peaceful march had been "completely overshadowed" by the actions of a minority who had "launched a ferocious and sustained attack on the police".

As the crowd began to disperse, mostly back to their coaches south of the Thames, small groups of activists marauded through the affluent areas surrounding Trafalgar Square, overturning cars before setting them alight and smashing-up and looting shops. Calm was not restored until 3am.

Now, watching huddles of tourists snapping each other in front of Nelson's Column, it's hard to imagine the mayhem of 20 years ago. And despite the protest's enduring association with violence, Chris Bambery sees it as a victory. "We felt we had dealt a big blow to Thatcher," he says. "When she's talked about on TV there are two images that are always used, the one of her being driven away in tears and the shots of Trafalgar Square. It was one of the defining moments of her tenure. My feelings were like everyone else's – we were no longer on the receiving end. We were taking a stand. It was a moment for people power in this country."