The only direct reporting from Saigon in the Observer on 17 March 1968 was on an inside page: a two-column dispatch by Gavin Young reflecting on the sobering effect on US officials and the military of Hanoi’s ferocious Tet offensive, which had ended the previous month. But Vietnam still permeated the paper, from the front-page lead on the world gold crisis, triggered above all by Lyndon Johnson’s huge spending on the war, and Bobby Kennedy’s announcement the previous day that he was going to challenge LBJ for the Democratic presidential nomination (only to be assassinated less than three months later), to Kenneth Tynan’s Shouts and Murmurs column from New York, recording his friend Gore Vidal saying that if the war continued after November’s elections “a change in nationality would be the only moral response”.

But there wasn’t, in the Observer or any other Sunday paper, a prediction of the mayhem that would ensue that very afternoon in Mayfair – “the Battle of Grosvenor Square”, as the paper would describe it a week later, before going on to suggest that not since the “fascist-communist fights of the 1930s” had the police been confronted with “sustained public violence” on the scale of that day’s demonstration against the US war in Indochina. Certainly the scenes in front of the US embassy had little of the decorousness familiar to those in the square – and there were many – who were veterans of CND Aldermaston marches or even of the Committee of 100’s sit-down anti-nuclear protests. James Callaghan, then home secretary, told the Commons the next day that 117 policemen had been injured, 246 protesters had been arrested and charged, and that 48 demonstrators had received medical care (St John Ambulance said it had treated 86 people on the spot).

If this was a turning point in demonstration style, it was also one for the police

It had all started peacefully enough at a rally in Trafalgar Square. The highlight was the appearance of the 31-year-old actress Vanessa Redgrave (not yet a Workers Revolutionary party member) , stunning in a deep orange cape and white crepe headband that the Morning Star informed its readers was “the national Vietnamese symbol of mourning”, reading out messages of support from some of the era’s leading cultural celebrities: film directors Sidney Lumet, Michelangelo Antonioni, Alain Resnais, Richard Attenborough and philosopher Bertrand Russell. Redgrave, armed with a letter of protest to hand in at the embassy, marched with writer Tariq Ali down Oxford Street towards the square; and so did about 15,000 others behind them. The 24-year-old Mick Jagger was somewhere in the crowd (it was the year of the Rolling Stones’ Street Fighting Man).

The real trouble started as the march turned down North Audley Street to face a phalanx of police at the entrance to the square. The police would say later they were only trying to channel the marchers into a specific route – but to most marchers it seemed that they were trying to block their entrance to the square. I have little memory of breaking through the cordon, or of how our little group of students found ourselves in the protest’s frontline, arms linked and with only a hedge, railings and a long line of police and the road between us and the embassy. Nor do I really recall when the police horses were brought into the square to confront the demonstrators.

But I do remember being pulled by the hair out of the crowd by a policeman; he pulled as a burly colleague assisted him by pushing. My then girlfriend Jane Steedman, later a teacher and education academic, and my flatmate Michael Harloe (who would become the long-serving vice-chancellor of Salford University) gallantly intervened to try to prevent the arrest. But to no avail. I was thrown over the fence and given a half-hearted kicking by a couple of other officers before being carted off to the waiting green bus.

A protester is bundled away outside the US embassy in Grosvenor Square. Photograph: Alamy

Also arrested and in the same bus was my other flatmate, Michael Rosen, the now celebrated children’s author, along with another friend, the late Dick Leith (future author of the seminal A Social History of English). The opening lines of a poem Rosen wrote for Oxford student magazine Isis about the demo were described by the late Christopher Hitchens, Rosen’s self-confessed rival for the role of Oxford enfant terrible, as “haunting”.

Beside generously giving me some undeserved, and brief, street cred among the comrades, Rosen’s poem went on to capture a mood among the “men in blue” who “got the boot [and truncheon] in” that afternoon. For if this was a turning point in demonstration style, it was also one for the police. Indeed, the acts of violence by a minority of demonstrators mainly followed the mounted police’s arrival into the confined space of the square’s gardens, where, as one of the very few press critics of the police, the Daily Express’s Alix Palmer, wrote: “They charged, they crushed, they trampled.”

When I was booked in at West End Central police station I was struck by the contrast between my young arresting officer and the desk sergeant, a figure much more reminiscent of Jack Warner’s affable copper in Dixon of Dock Green, the wildly popular TV series that had been running since 1955 but whose portrayal of a cosily humane force was already beginning to look outdated. The sergeant demanded that the loudly protesting PC talk to the police doctor before the lock-up. (The doctor – rightly – pronounced me fine, apart from a few bruises.)

The surprise came the following morning after a night in a cell crowded with euphoric detainees discussing, for some reason, China’s Boxer rebellion of 1900. Accepting unquestioningly the PC’s fictional testimony, the stipendiary magistrate found me guilty of assault on a police officer and handed down a month in prison suspended for three years. As miscarriages of justice go, it could hardly have been more trivial; but it was an early lesson on the reliability of police evidence in subsequent and vastly more important cases.

Elsewhere, by contrast, the police were lavishly praised. There were in fact two big Vietnam demonstrations we attended that year: the other, on 27 October, had a huge media build-up in contrast to the March one, against which it proved to be an anticlimax with only a brief flare-up at the end and the police reporting just five officers injured (along with 45 injured demonstrators and a mere 34 arrests). But the widespread coverage before the October protest closely reflected the reports after the first, with even an editorial in David Astor’s impeccably liberal Observer thundering that “to allege that the British police are violent is as dazzling a piece of hypocrisy as the big lies that Hitler once remarked deceive people more than small ones”.

Well, the deaths in 1970s demos of Kevin Gately and Blair Peach, the assaults on the innocent Birmingham Six, Orgreave, and several fatal police shootings were still in the future then.

But if the press coverage was almost uniformly hostile to the protesters, the attention given by the British government to what we thought of as the most important British event of the year, if not the decade, was minimal. Westminster was much more preoccupied with the talks on gold, Roy Jenkins’s imminent first and austere crisis budget and the sensational resignation of the foreign secretary, George Brown, the previous Friday after one drunken outburst too many. The only mention of the demo in the ministerial diaries is a brief reference by Barbara Castle to a conversation in which the US ambassador, David Bruce, had been “full of admiration for the British police”, but wondered if “we ought to continue allowing such demonstrations where there was organised violence reinforced by a large foreign element”. Castle, to her great credit, wrote: “I hope we don’t fall for any suggestion like this.”

However, if the demonstrations had near-zero direct political – and probably electoral – effect, they may have helped to shape the climate in subtler ways. If nothing else, they were part of the anti-Vietnam war tide that helped to alienate Harold Wilson from much of the leftwing support that had welcomed him to office in 1964 after “13 years of Tory misrule”, not only among students and the intelligentsia but within the Labour party itself. Though writing long before the Iraq war, historian Ben Pimlott’s description about what happened to Wilson, in his superb biography, is eerily similar to Tony Blair’s experience after 2003. It was because of Wilson’s refusal to speak against the Vietnam war that “writers, artists, scientists, expressed their disenchantment; intellectual fashion, most powerful of political motivators, moved away, and never returned”.

But was this “disenchantment” as fair on Wilson as it was on Blair? I don’t remotely regret our opposition to the Vietnam war, the greatest of several 20th-century US foreign policy disasters. But were we right to turn our wrath on the British prime minister? This was the year of the soixante-huitards; of revolutionary fervour in France and elsewhere in western Europe, including British universities the LSE, Essex, even Oxford; of the wave of anti-war opposition in the US that culminated at the Democratic convention in Chicago, where police clashed bloodily with demonstrators outside the International Amphitheatre.

Between the UK’s two big 1968 Vietnam demos, some of us joined the Revolutionary Socialist Students’ Federation, which despite its fearsome name was then in its infancy and a pretty ideology-light sub-Trotskyite group. At its June launch, the RSSF founders cited Wilson, along with the president of the recently occupied Sorbonne, as its chief bogeyman. On the marches we chanted not only “Hey, hey LBJ, how many kids did you kill today?” and “Ho-Ho-Ho Chi Minh”, but also “Where has Harold Wilson gone? Crawling to the Pent-ag-on!” (The 27 October Observer editorial had asked why, if the marchers were so anti-war, they cared more about the “Vietcong” than “the people of Prague”; but like good sub-Trotskyites, we also protested outside the Czech embassy when Moscow ordered in 100,000 Warsaw pact troops to crush Alexander Dubček’s Prague spring).

Under pressure greater than anything encountered by Blair in 2003, Wilson did not risk a single British soldier’s life

In fact, the historic injustice of stigmatising Wilson in this way is much clearer now than it was then, even to his supporters. What’s striking about Wilson’s handling of Vietnam was not that he bowed to overwhelming pressure from Washington but that he resisted it. And the pressures were huge; the UK had withdrawn its forces east of Suez, in a radical recognition of its post-imperial role. But in 1967 the French president, Charles de Gaulle, rebuffed Wilson’s attempt to join the European Common Market. Britain’s economy was woefully dependent on US help, not least in supporting a sterling under constant pressure – the government devalued in 1967.

Indeed, it may not be too fantastic to see similarities between the country then and what could yet befall it post-Brexit. As Pimlott says, Wilson “believed that it was impossible publicly to condemn a central part of US foreign policy … and at the same time accept American military, financial and political support”. Which makes it all the more remarkable that he stuck so consistently to his red line. He did provide limited technical and logistical help to the Americans, rather like the help those around Blair thought they might have to confine themselves to if parliament had voted against joining the Iraq invasion in 2003. But as Pimlott says: “[Wilson’s] response to pressure from Washington was to give the Americans everything they wanted, short of what they wanted most, which was British troops in Vietnam.” Under pressure vastly greater than anything encountered by Blair in 2003, Wilson did not risk a single British soldier’s life.

Unlike Blair, however, and certainly without being a pacifist, Wilson was instinctively anti-war, conditioned by his own nonconformist upbringing, reinforced by a disdain for military adventurism he had learned from his membership of the Labour grouping led in the 50s by Aneurin Bevan. Yet despite a certain personal antipathy by LBJ towards him, Wilson handled the US president with all his intelligence and skill, flattering him, sharing confidences – for example about De Gaulle – and somehow preserving Britain’s relationship with the US without conceding what LBJ “wanted most”. Instead, more than once he tried to act as a mediator to end the war, using Britain’s absence from the conflict and the UK’s joint chairmanship with Moscow of the Geneva conference; and at one point sending the leftwing MP Harold Davies to Hanoi in an admittedly unsuccessful peace bid.

Which raises the question of how a post-Brexit prime minister would react to a demand from, say, Donald Trump that Britain joins the US in some reckless military adventure in return for the bilateral trade deal it will desperately want if and when it leaves the EU. Indeed, there is a frightening plausibility about such a scenario. At first sight Wilson’s brilliance in keeping Britain out of a catastrophic American war without breaking the best of the ties between the two countries looks an encouraging precedent. But it’s hard to be sure that there is a post-Brexit prime minister waiting in the wings who could show the same combination of diplomatic skill, firmness, and complex understanding of the true national interest that Wilson did.

ARRESTS! PUBLISHED IN ISIS, 1 MAY 1968

The opening lines of a poem written by Donald Macintyre’s flatmate, Michael Rosen, for the Oxford student magazine Isis about the Grosvenor Square Vietnam demonstration. Both were arrested.

and old don came through the coach door

like a sack of coal and sat and shook

on the front seat wiping hair and blood off his eyes

and above, it was glass and steel

which is america thankyouverymuch

and away through our windows

friends in knots struggled with the thin blue line

of the stalwart boys of the neapolitan ice

thankyouverymuch

and then in through the door came dick

I’m alright he says

and you remember the bit behind the coach

where tired and footsore they got the boot in

thankyouverymuch said the ambassador today

so the door shuts like a school trip to hyde park

except that the man in blue on the door

said the answer would be to drop

a fuckin a-bomb on china or you lot huh

and the skin beneath don’s eyes stretches and shrinks ...